问题
选择题
(2013·安徽，21)From space, the earth looks blue.This is ________about seventy－one percent of its surface is covered by water.
A．why
B．how
C．because
D．whether
答案
答案：C
考查名词性从句。此处考查表语从句。根据句意：地球看起来是蓝色的。这是因为其表面71%是被水覆盖的。