单选题
For the creatures to become oil, it was necessary that they ______ between layers of rock for an enormous length of time.
A、
been imprisoned
B、
shall be imprison
C、
should be imprison
D、
will be imprisone
【正确答案】
C
【答案解析】
necessary 要求用虚拟，should +动词原形c. should be imprison
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