I think our duty to keep our environment clean and tidy.A. thisB. thatC. itD. one

问题补充：

I think our duty to keep our environment clean and tidy.

A. this

B. that

C. it

D. one

答案：

A. this

B. that

(答案→)C. it

D. one

解析：it常可以用作形式宾语或者形式主语，当think后面的宾语从句内容较长时，常常用it代替后面从句内容，做形式宾语，其它选项没有这个功能，故选C。

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