问题补充：
is known to us all that the World Expo took place in Shanghai.
A. It
B. As
C. Which
D. What
答案：
(答案→)A. It
B. As
C. Which
D. What
解析：考查it作为形式主语。真正的主语从句是后面的that the World Expo took place in Shanghai。
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