But for their generous help, the experiment______completed so quickly.A、shouldn’

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问题 But for their generous help, the experiment______completed so quickly.

选项 A、shouldn’t be
B、couldn’t have been
C、wasn’t
D、hadn’t

答案 B

解析要不是他们慷慨相助，试验不可能这么快完成。but for要不是…，位于句首句子用虚拟语气。couldn’t have been本不可能实现但却实现了，虚拟语气。

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Butfortheirgeneroushelp,theexperiment______completedsoquickly.A、shouldn’