Nhận biết (M1)

A trend away from stable, relatively high humidity in the Middle East over the past 8 million years can be inferred from the composition of cave formations dating to different periods. Cave formations from about 7 million years ago mainly consist of transparent columnar calcite, indicative of an underground water system regularly replenished by rainfall. Cave formations younger than approximately 1 million years, however, mainly consist of branching, opaque (and sometimes colorful) material, often with more frequent growth interruptions, indicating an intermittent water supply.

Which choice best describes the function of the underlined portion in the text as a whole?

• A.

  They describe divergent features of cave formations that when taken together imply decreasing variation in Middle Eastern rainfall over time.

• B.

  They identify different characteristics of cave formations that when considered together provide evidence for a long-term shift toward lower and more variable humidity in the Middle East.

• C.

  They provide evidence from different Middle Eastern cave formations that substantiates the claim that the region experienced a long period of relatively high humidity.

• D.

  They draw a contrast between Middle Eastern cave formations that is suggestive of geographic differences in the underground water supply.