I tried to persuade him to give up the foolish idea but in and now he begins to regret.

问题补充：

I tried to persuade him to give up the foolish idea, but in , and now he begins to regret.

A. vain

B. turn

C. return

D. case

答案：

(答案→)A. vain

B. turn

C. return

D. case

解析：in vain意为“徒劳地”；in turn意为“依次，轮流”；in return意为“作为回报”；in case意为“万一，以防”。根据句意“我原来极力劝他放弃那个愚蠢的想法，他不听，现在他却开始后悔了”可知，答案为A。