is more important is as the earth cooled down, water began to appear on its surface.
A. What /that
B. That/what
C. What / what
D. That / that
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(答案→)A. What /that
B. That/what
C. What / what
D. That / that
解析：本题考查名词性从句。第一个空是一个缺少空语的主语从句，第二空是一个表语从句，表语从句又是一个as引导的时间状语从句。因此答案是A。