Is dangerous to swim in the river alone?A. itB. thisC. thatD. he

问题补充：

Is dangerous to swim in the river alone?

A. it

B. this

C. that

D. he

答案：

(答案→)A. it

B. this

C. that

D. he

解析：本句it为作形式主语，不定式短语放在后面作真正主语。