单项选择题
We have kept（）our friendship by writing to each other, so we are true friends.
A.back B.up C.off D.down
单项选择题 We failed in our（）to climb the mountain.
单项选择题 If you want to learn something, you had better pay（）in class.
单项选择题 In addition to rice, we need to（）our diet with fish, meat and vegetable.
