51. 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?
(A) It indicated that lighter isotopes of oxygen predominated at certain times.
(B) It had far more gaps in its sequence than the record taken from rocks on land.
(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 for only a million years.
record taken from rocks.
A describes a circumstance that is in fact true, since oxygen 16 is the lighter isotope and,
according to the passage, “Almost all the oxygen in water is oxygen 16”(line 17). This fact clearly
has not compromised the usefulness of the ocean’s isotopic record as an indicator of climatic shifts.
Likewise, E would not diminish its usefulness, since isotopic records showing “fluctuations in
global ice volume over the past several hundred thousands years” have been sufficient to
determine a meaningful pattern (lines 49-53). If C were shown to be true, Milankovitch’s theory
would be weakened, but this would not diminish the value of the isotopic record. If D were true,
researchers would need to accommodate the inconsistency described in evaluating the isotopic
record, but, again, this would not compromise the usefulness of the record itself.
还是不懂D为什么不可以?? | 
