Is necessary to complete the design before National Day?A. thisB. that　C. it　D. he

问题补充：

Is necessary to complete the design before National Day?

A. this

B. that

C. it

D. he

答案：

A. this

B. that

(答案→)C. it

D. he

解析： it作形式主语，代指后面的不定式。