这题应该选哪个,为什么?

B
此题考查虚拟语气的倒装.
虚拟条件句的从句部分如果含有were,should,或had,可将if省略,再把were,should或had 移到从句句首,实行倒装.
Were they here now,they could help us.
　=If they were here now,they could help us.
Had you come earlier,you would have met him
　=If you had come earlier,you would have met him.
Should it rain,the crops would be saved.
　=Were it to rain,the crops would be saved