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.

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


Experts, could you please shed some light on this question?

Regards,
Amit