及解析Lecture4，来认真分析一下吧!
Lecture 4
1. What does the professor mainly discuss? 【主旨题】
A Methods of converting radio waves into sound waves
B Features of different types of electromagnetic radiation
C The various paths that very-low-frequency waves follow on Earth
D The emission and detection of very-low-frequency waves
答案：A
破题关键词汇：
解析：(从24秒开始，原文重现：Are you going to talk at all about the difference between radio waves and sound waves?
Uh, Ok. That might be a good place to start actually.)
此处需要结合全文判断，并且利用排除法排除干扰选项。首先学生提问引出主要话题方向是与无线电波和声波的区别有关，教授表示这是一个很好的话题开头。选项中没有直接的答案，因此结合全文，B选项的Features of different types、 C的various paths、 D的emission and detection都是未详细展开的内容，不属于主旨，且下文都与sound有关，因此正确答案为A。
2. What is one difference between radio waves and sound waves that the professor emphasizes? 【细节题】
A Radio waves have a lower frequency.
B Water stops radio waves from spreading but does not stop sound waves
C Unlike sound waves, radio waves can travel outside Earth's atmosphere.
D Naturally occurring radio waves are difficult to detect on Earth at night.
答案：C
破题关键词汇：difference， radio waves and sound waves
解析：(从58秒开始，原文重现：Radio waves, on the other hand, are fundamentally different from sound waves. They are electromagnetic. They result from oscillations of the electromagnetic field and don't need a physical medium, so they, like other types of electromagnetic wave, can travel basically anywhere, through a vacuum, or through atmosphere or water.)
此处问到无线电波和声波的区别，教授提到无线电波可以travel basically anywhere，对应选项C。
3. What explanation does the professor give for the constant occurrence of VLF emissions on Earth? 【细节题】
A At any given time, some part of the world is experiencing sunrise or sunset
B Waveguides constantly form in the atmosphere.
C Earth’s magnetosphere directs interplanetary waves toward Earth’s surface.
D Lightning occurs constantly on the planet.
答案：D
破题关键词汇：VLF emissions
解析：(从1:55秒开始，原文重现：Um, on Earth, the main source of naturally occurring VLF emissions is lightning, which generates a pulse of radio waves every time it flashes. ;从2:23秒开始，原文重现：So we can pick up emissions from lightning that's far away. So actually, you can pretty much listen to them all the time because lightning strikes Earth constantly, about a hundred times per second.)
此处提到VLF释放的主要来源是闪电，可以经常听见闪电在地球上发生，对应选项D。
4. Why are sunrise and sunset the best times to listen to VLF signals? 【细节题】
A Because thunderstorms are most likely to occur then
B Because radio waves travel through natural waveguides then
C Because higher-frequency signals are less active then
D Because temperatures are not extremely high or low then
答案：B
破题关键词汇：sunrise and sunset
解析：(从3:09秒开始，原文重现：He says the best time to listen for them is around sunset or sunrise. That's when there're natural waveguides in the local atmosphere.)
此处教授提到他同事给出的说法，听到VLF的最佳时机大约是在傍晚或日出，这个时间当地的大气中含有自然的电磁波导，对应选项B。
5. Why does the professor discuss whistlers and tweeks? 【目的题】
A To illustrate that the path a VLF wave travels can affect the sound it makes on a radio
B To point out that VLF waves can affect the sounds heard on a household or car radio
C To describe how a colleague discovered the origin of VLF waves
D To clarify the difference between VLF waves and other kinds of waves
答案：A
破题关键词汇：whistlers and tweeks
解析：(从3:32秒开始，原文重现：They make a path for radio waves to follow in our atmosphere. These natural waveguides occur when the Sun is rising or setting, which makes sunrise and sunset good times to pick up VLF emissions.
Now, there are a few different sounds that you can hear on a VLF receiver, because when lightning strikes the radio waves travel different distances and in different ways before they reach the receiver.)
此处需要结合上下文倒推，whistlers和tweeks是两个例子，用来解释说明上文提到的收音机可以听到a few different sounds，接着详细解释了whistlers和tweeks的区别。对应选项A。
6. What does the professor imply when he says this: 【重听题】
A He needs to think before he can answer the woman's question.
B The woman has underestimated how often VLF waves can be detected.
C The woman does not realize that waiting for a thunderstorm can take a long time.
D The woman does not understand the relationship between thunderstorms and lightning.
答案：B
破题关键词汇：
解析：(从2:09秒开始，原文重现：Since you almost always get lightning with thunderstorms. We can pick up VLF waves pretty often, right? You just have to wait until there is a thunderstorm.
Ah, do you? Have to wait? VLF receivers are very sensitive and VLF waves travel very far. So we can pick up emissions from lightning that's far away.)
此处学生先提出需要VLF声波可以等雷暴天气发生，教授对此进行了反问，且教授回答中强调了travel very far和that's far away，暗示VLF声波会传播很远，并不是很轻易可以等到的，对应选项B。
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