原题
In cave systems in the Umm Er Radhuma Formation of modern Saudi Arabia, Monika Markowska and colleagues found that from 60,000 to 530,000 years ago, both calcite and gypsum minerals formed in speleothems (stalactites, stalagmites, flowstones, and other cave formations); after this period, though, only gypsum deposits formed. The process of calcite formation requires sufficient vegetation and soil to produce carbonic acid that dissolves limestone, while gypsum deposits primarily form through evaporation processes and are typical in arid environments. Interestingly, there is some evidence of local groundwater reservoir increases after 60,000 years ago. The concurrent shift in speleothem mineral deposition, however, suggests that ______
Which choice most logically completes the text?
Abeginning 60,000 years ago, local precipitation fell to levels too low to maintain the vegetation necessary for gypsum formation.
Bhigher water levels in groundwater reservoirs were not due to increased local precipitation.
Cdeclining local aridity after 60,000 years ago facilitated the shift in groundwater reservoir levels.
Dchanges in the mineral composition of groundwater reservoirs were independent of local precipitation patterns.
正确答案：B
解题思路
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