All his attempts to unlock the door were because he was using the wrong key.A. in returnB

问题补充：

All his attempts to unlock the door were, because he was using the wrong key.

A. in return

B. in vain

C. in doubt

D. in time

答案：

A. in return

(答案→)B. in vain

C. in doubt

D. in time

解析：考查短语辨析。in return 作为回报，in vain 白费力，白辛苦，in doubt 怀疑的，in time 及时，句意：打开这个门的所有努力白费了，因为他用错了钥匙。所以选B。