His sister left home in 1998,and_______ since .
| A．had not been heard of | B．has not been heard of |
| C．had not heard of | D．has not heard of |
I won’t call you, __________ something unexpected happens.
| A．unless | B．whether | C．because | D．while |
He was told that it would be at least three more months __________ he could recover and return to work.
| A．when | B．before | C．since | D．that |
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 |
You will be successful in the interview __________ you have confidence.
| A．before | B．once | C．until | D．though |
—Where’s that report?—I brought it to you __________ you were in Mr. Black’s office yesterday.
| A．if | B．when | C．because | D．before |