这是不是OG题目的里面的一个bug啊，求大神解答！！

leeeeee

Milankovitch proposed in the early twentieth
century that the ice ages were caused by variations
in the Earth’s orbit around the Sun. For some
time this theory was considered untestable,
largely because there was no sufficiently precise
chronology of the ice ages with which the orbital
variations could be matched.
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. Almost
all the oxygen in water is oxygen 16, but a few
molecules out of every thousand incorporate 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 can be determined
by analyzing ocean sediments 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. The dated
isotope record shows that the fluctuations in
global ice volume over the past several hundred
thousand years have a pattern: an ice age occurs
roughly once every 100,000 years. These data have
established a strong connection between variations
in the Earth’s orbit and the periodicity of the ice
ages.
However, it is important to note that other
factors, such as volcanic particulates or variations
in the amount of sunlight received by the Earth,
could potentially have affected the climate. The
advantage of the Milankovitch theory is that it
is testable; changes in the Earth’s orbit can be
calculated and dated by applying Newton’s laws
of gravity to progressively earlier configurations
of the bodies in the solar system. Yet the lack of
information about other possible factors affecting
global climate does not make them unimportant.
75. It can be inferred from the passage that calcium
carbonate shells
(A) are not as susceptible to deterioration as rocks
(B) are less common in sediments formed during an
ice age
(C) are found only in areas that were once covered
by land ice
(D) contain radioactive material that can be used to
determine a sediment’s isotopic composition
(E) reflect the isotopic composition of the water at
the time the shells were formed

我就想问75题~~
这道题给出的答案是选E，OG的解释是说根据文章说，shell形成的时候是从周围的海洋中吸收氧原子的，所以理所当然从这个分析出的 composition是shell形成的时候。
但是看文中高亮部分说反映的冰川构成是在sediment was laid down的时候，但是shell形成的时候不是应该和setiment laid down 的时候不一样吗？。。。。可能shell形成了很久，然后才laid down
要不然是我理解有误！！！求大神解答！！