from space the earth is blue.A. When seeingB. When seeC. When we see itD. When to see

问题补充：

from space, the earth is blue.

A. When seeing

B. When see

C. When we see it

D. When to see

答案：

A. When seeing

B. When see

(答案→)C. When we see it

D. When to see

解析：本题的含义是从太空中看地球，地球是蓝色的，本题是由when引导的时间状语从句才符合句意，故本题选C。