—Why didn't you try your best to get on the bus?—I tried to, but it had started moving ________ I could get on it.
Sometimes parents find ______ hard to communicate with their children.
Nowadays scientists are working hard to find out ______ to do with air pollution.
Good teachers can usually give the students______ advice on their study.
Chinese people have been using paper for over______ years.
Mother came into my study with a cup of milk in one of her hands, and an apple in ______.