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

网友回答

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。