【题目】It was ________ that we tried to make him change his mind.
A. paid off B. in vain
C. took off D. in effect
在线课程【答案】B
【解析】考查动词短语及介词短语辨析。A. paid off “付清；成功，有回报（Sth pay off.）”；B. in vain“徒劳”；C. took off “脱下；起飞”D. in effect“（指规则、法律等）生效，实际上”，所以该句It为形式主语，that引导真实的主语从句，该句句意：“我们尽力使他改变主意是徒劳的。”，故正确答案为B.