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
网友回答
A. that
(答案→)B. that as
C. which
D. which as
解析：考查表语从句。本题中的that引导的是表语从句，表语从句中有一个时间状语从句。句意：更重要的是随着地球的冷却下来，水也开始出现在地球的表面上了。故B正确。