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

A. that           B. this          C. it             D. one

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

A. that

B. this

C. it's

D. one

1 

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

A. this       B. that      C. it  D. one