【题目】I have done everything possible to prevent him from running the risk but ______.
A.in time
B.in case
C.in vain
D.in need

【答案】C
【解析】考查固定搭配。A. in time及时；B. in case以防万一；C. in vain徒劳；D. in need需要。句意：我尽了一切可能(我已经做了一切可能做的事情)来阻止他去冒险，但这只是徒劳的。故选C。