If it were to snow tomorrow,they ____ out.

虚拟语气的搭配都是固定的.你的你的主句是If it were,那从句就只能是would+动词原形.
与..事实相反 If从句 主句
过去 Had done Would have done
现在 Were/did Would* do
将来 Should do/were/were to do Would* do

例句：If there hadn’t been much rain in spring, we would have had a good harvest now. [过去]
If he smoked less, his cough might be well soon. [现在]
If the lecturer should be late, you would have to make a speech first. [将来]

既然是虚拟语气，就该选A嘛，honorousjack粗心了，嘿嘿。

A
if引导条件状语从句，前后时态应该一致，所以排除D
又因为题中有tomorrow，所以应该是过去将来时，所以选A

A
虚拟语气

A
虚拟语气，与将来事实相反，主句用would + 动词原形

A 虚拟语气 阐述未发生的事件。用将来时态