—Your sister’s plan is not quite practical.Why not try to persuade her to give it up?—My s

问题补充：

—Your sister’s plan is not quite practical.Why not try to persuade her to give it up?

—My sister is very stubborn，so it isto try to change her mind.

A. by chance

B. in vain

C. on purpose

D. for sure

答案：

A. by chance

(答案→)B. in vain

C. on purpose

D. for sure

解析：

句意为：——你妹妹的计划不是很切合实际。为何不劝她放弃？——我妹妹很固执，所以要她改变主意根本就是白费力气。in vain意为“枉费心机，白费力气”。by chance碰巧，凑巧；on purpose故意地；for sure毫无疑问。