问题
选择题
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
答案：B
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。