虚拟语气,根据虚拟语气的结构 ：
对过去事实相反的假设,
条件从句：过去完成时
主句：should（would）等+have +过去分词 ,根据句子后半部分使用的句型,判断出得使用过去完成时,排除BD
if 引导的条件句中,C缺少主语.
A是虚拟语气倒装句.
虚拟条件句的从句部分含有were,should,或had时,可省略if,再把were,should或had 移到从句的句首,实行倒装.例如：
Were they here now,they could help us.=If they were here now,they could help us.他们现在在的话,就会帮助我们了.
Had you come earlier,you would have met him.=If you had come earlier,you would have met him.你来得早一点,就碰到他了.