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第一篇
TOPIC Indus Civilization衰落
本文分成了三大段。首先讲Indus文明衰落与某VA无关，因为VA是其衰落之后才来的。它与地理(第一段)、气候等都有关系。地理方面涉及tectonic plates, flooding以及某G River的变化;气候方面，多个时期降雨都有变化。第三段讲由于食物短缺，最后小城市过了一段后逐步变成农村了。
解析：本文属历史类话题，讲述早期文明发展。文章结构主体为因果结构。本文分三大段，是
Decline Of Harappan Civilization:
The Indus Valley Civilization dated from 3300 1300 BCE that was located in the north western region of the Indian Subcontinent. Flourishing around the Indus River basin, the civilization primarily centered along the Indus and the Punjabregion, extending into the Ghaggar-Hakra River valley and the Ganges-Yamuna Doab, encompassing most of what is now Pakistan, the western states of modern-day India, as well as extending into southeastern Afghanistan, and the eastern most part of Balochistan, Iran. The mature phase of this civilization is known as the Harappan Civilization, as the first of its cities to be unearthed was the one at Harappa, excavated in the1920s in what was at the time the Punjab province of British India (now in Pakistan).
The decline of Harappan culture is difficult to explain. During its late phase between 2000 and 1700 BC 'The Indus Valley Civilization as a distinct entity gradually ceased to exist'. Historians have different opinions regarding the causes of the decay and disappearance of the Harappan culture. Various causes have been ascribed for its weakening and then decay: Increase in rainfall, earthquake, decrease in fertility of soil, floods, Aryan invasion, disease etc. Mortimer Wheeler pointed out that the Harappan culture was destroyed by the Aryans. The Aryans were more skilled at warfare and were powerful than the Harappans. But there is very little evidence on this opinion.
Another school of thought relates the demise of the Indus valley civilization to have been brought about by a major tectonic shift that caused continuous floods of this area. Research has proved that the decline of the glorious Harappan culture was due to a variety of factors, both manmade and natural.
In the beginning of the second millennium BC, there were great changes in the environmental conditions-the climate changed and large parts of the plains were flooded when tectonic changes threw up a dam in the lower Indus Valley. There were also other socio-economic factors that contributed to the decline. Agricultural production declined with the changes in the climate and the big cities could no longer sustain themselves. People from the major centers perhaps left for the smaller outposts and slowly riveted back to village life when they could no longer maintain the prerequisites of an urban existence. The causes of its decline are not certain. However, the physical existence of the civilization ended due to various factors: Economical changes led to the decline of land and agriculture, thereby enforcing the need to evacuate to other area might have been the reason for the disintegration of the Indus Valley. Shifts in the monsoon pattern and changes in temperature led to the areas more arid. Increase in Population, excessive deforestation, decline in agriculture etc. might have created economic problems leading to the gradual decay of the culture. The marked decline in the quality of building and town planning indicates that the authorities were losing control. The changes in the river flow pattern and correspondent widespread flooding would have disrupted the agricultural base.#p#副标题#e#
第二篇
TOPIC 美国国家公园的发展
首先讲开发公园，然后开发railway，打广告吸引游客，公园与铁路共同发展。接下来highway发展起来，国家机构也来管理这些发展，投入资金，并建立国家公园办公室。一系列数据表明越来越多的人来国家公园，这些对国家公园发展产生影响。
解析：本文不属于
National Parks - A Brief History
One of the first people generally credited with conceptualizing a "national park" was George Catlin (1796-1872), a self-taught artist who traveled extensively among the native peoples of North America, while sketching and painting portraits, landscapes, and scenes from daily Indian life. On a trip to the Dakotas in 1832, he worried about the impact of America's westward expansion on Indian civilization, wildlife, and wilderness. They might be preserved, he wrote, "by some great protecting policy of government . . . in a magnificent park . . . . A nation's park, containing man and beast, in all the wild and freshness of their nature's beauty!"
The idea had gained some acceptance years later, when in 1864 Congress donated Yosemite Valley to California for preservation as a state park. The establishment of Yellowstone National Park by act of Congress on March 1, 1872, for the first time signified that public lands were to be set aside and administered by the federal government "for the benefit and enjoyment of the people." In 1891, President Harrison established Yellowstone Timberland Reserve as the nation's first forest reserve, and in 1903 President Roosevelt established Pelican Island in Florida as the first national wildlife refuge. There was still no real system of national parks in the United States until August 25, 1916, when President Woodrow Wilson signed the Organic Act creating the National Park Service (NPS). Established under the umbrella of the Department of the Interior, the NPS was responsible for protecting the 40 national parks and monuments then in existence.
In the years that followed, additional national parks and monuments (mostly in the western states) were administered by the NPS, while other monuments and natural and historical areas were administered as separate units by the War Department and the Forest Service of the Department of Agriculture. No single agency provided unified management of the varied federal parklands. An Executive Order in 1933 transferred 63 national monuments and military sites from the Forest Service and the War Department to the National Park Service. This action was a major step in the development of today's truly national system of parks-a system that includes areas of historical, cultural, scientific, and scenic importance.
In 1970, Congress declared in the General Authorities Act that all units of the system have equal legal standing in a national system. Areas of the National Park System, the act states,
"though distinct in character, are united through their inter-related purposes and resources into one national park system as cumulative expressions of a single national heritage; that, individually and collectively, these areas derive increased national dignity and recognition of their superb environmental quality through their inclusion jointly with each other in one national park system preserved and managed for the benefit and inspiration of all people of the United States..."
Additions to the National Park System are now generally made through acts of Congress, and national parks can be created only through such acts. But the President has authority, under the Antiquities Act of 1906, to proclaim national monuments on lands already under federal jurisdiction. The Secretary of the Interior is usually asked by Congress for recommendations on proposed additions to the System. The Secretary is counseled by the National Park System Advisory Board, composed of private citizens, which advises on possible additions to the System and policies for its management.
Though we use the term "national park" in a general sense when referring to the individual units within the National Park System, the classification system used by NPS actually encompasses 19 separate designations. Some are descriptive listings, such as lakeshores, seashores, and battlefields, but others also include titles that can't be neatly categorized because of the diversity of resources within them. The National Park System today comprises 376 areas covering more than 83 million acres in 49 States, the District of Columbia, American Samoa, Guam, Puerto Rico, Saipan, and the Virgin Islands. These areas are of such national significance as to justify special recognition and protection in accordance with various acts of Congress.#p#副标题#e#
第三篇
TOPIC Tide Pool
首先说Tide Pool定义，里面充满水，看似对生物好，其实生活在里面的生物更危险(有句子简化题)。然后讲Tide Pool三个物理化学影响因素：第一点，温度。因为和ocean相比，水越多，缓冲效果越明显，而这个pool里面温差很大，随涨潮和落潮，一会儿干一会儿湿，温差自然大。第二点，盐度变化也很大，太阳出来，海水蒸发，有盐析出，水的盐度大;而下雨的时候，水盐度低。第三点，氧气，生物越多，越会受到氧气压力的影响。
解析：本文属地理地质类话题，介绍某种水体。文章结构非常清晰明了，分三点影响因素介绍，属于典型的分类型文章。
Tide Pool
Tide pools are those areas of the coastline that are both exposed to the air and covered by water, depending on the tides. Also called the intertidal zone, a number of abiotic factors influence the unique ecosystem found in these areas. Because of the constantly changing nature of tide pools, the organisms that have made their homes there need to be adapted to deal with that change.
Shape
A large, shallow pool has a large surface area to volume ratio or a lot of the pool is exposed at the surface. A small, deep pool has a small surface area to volume ratio. Shape and volume become important when we consider the other factors that contribute to stress in a tide pool.
Temperature
The temperature of tide pools is affected by air temperature and direct sunlight on the pool. Shallow pools with large surface areas are likely to be more affected by temperature. The major impact of temperature changes on organisms is a change in rates of photosynthesis and respiration. For every 10 degrees centigrade, photosynthesis increases by one and a half times and respiration by two to three times. This causes a build up of by-products, such as carbon dioxide and oxygen, within the water.
Oxygen
Photosynthesis in plants, especially algae, cause oxygen concentrations to peak in the afternoon. At night, available oxygen is depleted rapidly and animals migrate to the surface. A pool with a large surface area to volume ratio will allow more oxygen to diffuse into the water from the air.
Salinity
The sun and wind evaporate water from the surface of pools. In isolated tide pools, this causes an increase in salinity (salt-concentration). Heavy rainfall can lower salinity rapidly, however the less dense freshwater often forms a layer above the salty water. The mixing is not always uniform and animals and plants in the pool's depths may remain unaffected. Some species are able to survive a wide salinity range.