What is even more important is the earth cooled down, water began to appear on its surfac

What is even more important is the earth cooled down, water began to appear on its surface.
A. that
B. that as
C. which
D. which as

A. that (答案→)B. that as C. which D. which as解析：试题分析:考查名词性从句。根据结构可知is 后面是表语从句，表语从句中又有状语从句，所以需要两个连接词，一个引导表语从句，一个引导状语从句。which引导从句需做成分，而表语从句中不缺少成分，所以B选项正确。句意：甚至更重要的是当地球冷却的时候，水开始在地球表面上出现了。

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