He had to pause from time to time to wipe the sweat from his forehead, because the air-conditioning system _________.

Many beautiful fish are disappearing____ the severe(严重的) pollution.

Having a trip abroad is certainly good for the old couple, but it remains_________ whether they will enjoy it.

Here ______ in the classroom.

You look worried，Laura.What’s wrong—I have some problems ________ my schoolwork.