That tragedy distressed me so much that I used to keep indoors and go out only ______ necessity.

In the airport, I could hear nothing except the roar of...

单项选择题In the airport, I could hear nothing except the roar of aircraft engines which ______ all other sounds.

Desert plants ______ two categories according to the wa...

单项选择题Desert plants ______ two categories according to the way they deal with the problem of surviving drought.