求问CD上的NN 一道OG17的阅读题 跪求

To establish such a chronology it is necessary to determine the relative amounts of land ice that existed at various times in the Earth's past. A recent discovery makes such a determination possible: relative land-ice volume for a given period can be deduced from the ratio of two oxygen isotopes, 16 and 18, found in ocean sediments.（做到这道题才真正明白确实不能像雅思定位一样找同义替换，在这段找了半天都没有答案,就是因为看到这句话，认为是定位的内容） Almost all the oxygen in water is oxygen 16, but a few molecules out of every thousand incorporate（包含）the heavier isotope 18. When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it. Because heavier isotopes tend to be left behind when water evaporates from the ocean surfaces, the remaining ocean water becomes progressively enriched in oxygen 18. The degree of enrichment（丰富） can be determined by analyzing ocean sediments（沉积物）of the period, because these sediments are composed of calcium carbonate shells of marine organisms, shells that were constructed with oxygen atoms drawn from the surrounding ocean. The higher the ratio of oxygen 18 to oxygen 16 in a sedimentary specimen, the more land ice there was when the sediment was laid down.

As an indicator of shifts in the Earth's climate, the isotope record has two advantages. First, it is a global record: there is remarkably little variation in isotope ratios in sedimentary specimens taken from different continental locations. Second, it is a more continuous record than that taken from rocks on land. Because of these advantages, sedimentary evidence can be dated with sufficient accuracy by radiometric methods to establish a precise chronology of the ice ages.

2.It can be inferred from the passage that the isotope record taken from ocean sediments would be less useful to researchers if which of the following were true?

从海洋沉淀里取出来的同位素可能对研究人员不那么有用如果下列哪一项是正确的？

A.It indicated that lighter isotopes of oxygen predominated（占优势） at certain times.

B.It had far more gaps in its sequence than the record taken from rocks on land.（正确答案）

C.It indicated that climate shifts did not occur every 100,000 years.

D.It indicated that the ratios of oxygen 16 and oxygen 18 in ocean water were not consistent with those found in fresh water.

E.It stretched back （回溯到）for only a million years.