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题库 / Prep2012-SC-35
The possible long-term health risks to airline personnel who repeatedly cross multiple time zones has been a concern since the first jet transports went into service in the 1950s.
who repeatedly cross multiple time zones has been a concern since the first jet transports went
who are repeatedly crossing multiple time zones has been a concern since the first jet transports that went
who repeatedly cross multiple time zones have been a concern since the first jet transports went
repeatedly crossing multiple time zones has been a concern since the first jet transports went
repeatedly crossing multiple time zones have been a concern since the first jet transports going
题目分析：
since是连词，身后应该连接一个从句，而(B)(E)中since身后均不是完整句，可以排除。在(A)(D)中，主句主语The possible long-term health risks和谓语动词has been主谓不一致，可以排除(A)(D)，答案为(C)。
选项分析：
略
since可以做连词，也可以做介词。
另外，repeatedly crossing是不是也有问题呢？crossing无时间终结点。
我感觉没问题。。都是先行词省略了
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