The police still haven’t found the lost child, but they’re doing all they .
Some students often listen to music _____ classes to refresh themselves.
.—____ when has the country been open to international trade? —1978, I suppose.
I think he is taking an active part in social work. I agree with you ______ .
Experts have been warning _____ of the health risks caused by passive smoking.
He was told that it would be at least three months ___ he could recover and return to work.