I think that ____ very difficult to learn.

Twenty years _____ passed since I came to this school, but I feel twenty years _____a short time in my life. 

We all think we are tired after the long work. Let，s stop      a rest.

One or two days ____quite enough to complete the work. ___ you or he going to work with me?

Since the forest has been destroyed by a big fire, a large ______ of trees ______ to be planted on the mountain．