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??

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

大致意思是：如果D成立的话，则海水和淡水的O同位素比例存在the inconsistency (不一致)，那么这种不一致肯定需要科学家进一步accommodate(解释)了。但即便如此，这种海水和淡水的不同（以及对这种不同所进行的解释）不会对the usefulness of the record (同位素记录本身的有效性)产生不利影响。

也就是说，即便真有不同，海洋沉积物的同位素记录仍可以对了解冰川周期带来帮助，而且并不会因为海水、淡水的同位素比例不同而降低海洋沉积物的同位素记录的这种有效性。因为选项中"海水、淡水的同位素比例不同"和题目中“降低海洋沉积物的同位素记录的有效性”这两个命题之间无关。