单选题 The supervisors could have prevented this problem______about it beforehand.
  • A. if they knew
  • B. had they known
  • C. if had they known
  • D. if they would know
【正确答案】 B
【答案解析】[解析] 虚拟条件句三种方式表示如下： [*] 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。