The supervisors could have prevented this problem______about it beforehand.

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问题 The supervisors could have prevented this problem______about it beforehand.

选项 A、if they knew
B、had they known
C、if had they known
D、if they would know

答案B

解析 如果if条件句中含有were,should,had等词,可以省去if,并把were,should,had 移到从句首,意思不变。
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