If it ___ for the help of our teacher ,we would not have made so much progress.A were not B has not been C isn't D had not been为什么

问题描述:

If it ___ for the help of our teacher ,we would not have made so much progress.
A were not B has not been C isn't D had not been
为什么

D
虚拟语气,讲的是过去的事