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

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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。