We are going to have a barbecue _______ it rains .

We are going to have a barbecue _______ it rains .
A.if not B.when C.except that D.unless
选哪一个啊~
C为什么不行啊?
B肯定不行，现在就是不确定C和D
是不是应该这样理解啊？Unless引导的条件状语从句遵循主将从现的原则，except that后引导从句，但是应该是except that it will rain.所以选择D。