---I think English is ____unimportant subject.---I don’t think so. It is ____useful tool when we talk with foreigners.
I got a letter this afternoon. It was ____ my foreign friend.
I’m all ____ sea and I’ve no idea how to fix my pencil box.
We built this factory ______ top speed and minimum(最少的) cost.
There is a big and beautiful lake ______ the foot of the hill.
We go _____ duty at 8 a.m.