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。