Students should do everything they can ________ their study.
In those days bikes were not ______ used because few people could afford (负担起) to buy one.
Jenny keeps two pets. One is a dog, ______ is a rabbit.
Which of the following signs is used to tell people to save water?
He was ______ tired ______ go on walking.
Have you ever read books ______ by Lu Xun?