The Earth’s magnetic field is generated as the molten iron of the Earth’s outer core revolves around its solid inner core. When surges in the molten iron occur, magnetic tempests are created. At the Earth’s surface, these tempests can be detected by changes in the strength of the Earth’s magnetic field. For reasons not fully understood, the field itself reverses periodically every million years or so (or so: 大约). During the past million years, for instance, the magnetic north pole has migrated between the Antarctic and the Arctic.
Clearly, geophysicists who seek to explain and forecast changes in the field must understand what happens in the outer core. Unlike meteorologists, however, they cannot rely on observations made in their own lifetimes. Whereas atmospheric storms arise in a matter of hours and last for days, magnetic tempests develop over decades and persist for centuries. Fortunately scientists have been recording changes in the Earth’s magnetic field for more than 300 years.
24. In the passage, the author is primarily concerned with
(A) analyzing a complicated scientific phenomenon and its impact on the Earth’s surface features
(B) describing a natural phenomenon and the challenges its study presents to researchers
(C) discussing a scientific field of research and the gaps in researchers’ methodological approaches to it
(D) comparing two distinct fields of physical science and the different research methods employed in each(B)
(E) proposing an explanation for a geophysical phenomenon and an experiment that could help confirm that explanation
25. The passage suggests which of the following about surges in the Earth’s outer core?
(A) They occur cyclically every few decades.
(B) They can be predicted by changes in the Earth’s inner core.
(C) They are detected through indirect means.
(D) They are linked to disturbances in the Earth’s atmosphere.(C)
(E) They last for periods of about 1 million years.
26. It can be inferred from the passage that geophysicists seeking to explain magnetic tempests ought to conduct research on the Earth’s outer core because the Earth’s outer core
(A) is more fully understood than the Earth’s magnetic field
(B) is more easily observed than the Earth’s magnetic field
(C) has been the subject of extensive scientific observation for 300 years
(D) is involved in generating the Earth’s magnetic field(D)
(E) reflects changes in the inner core caused by magnetic tempests
27. In the second paragraph, the author is primarily concerned with
(A) stating a limitation that helps determine a research methodology
(B) making a comparative analysis of two different research methodologies
(C) assessing the amount of empirical data in the field of physical science
(D) suggesting an optimistic way of viewing a widely feared phenomenon(A)
(E) describing a fundamental issue and discussing its future impact on society