Lei Feng is always ready to help others when they are in trouble and he never _______ their request.
| A．turns up | B．turns over | C．turns in | D．turns down |
He was told that it would be at least three months ___ he could recover and return to work.
| A．when | B．before | C．since | D．that |
Scientists are convinced ___ the positive effect of laughter ___ physical and mental health.
| A．of; at | B．by; in | C．of; on | D．on; at |
We had to wait half an hour _______ we already booked a table.
| A．since | B．although | C．until | D．before |
Help others whenever you can ______ you will make the world a nicer place to live in.
| A．and | B．or | C．unless | D．but |
Most birds find it safe to sleep in the trees, but ____ they have eggs or young chicks, they don’t use a nest.
| A．why | B．how | C．unless | D．where |