15.A number of books ______ been stolen from the library since the beginning of this term and the number _____ as large as 500.
| A．have; are | B．has; is | C．has; are | D．have; is |
There were some chairs left over________everyone had sat down.
| A．when | B．until |
| C．that | D．where |
She had just finished her homework________her mother asked her to practise playing the piano yesterday.
| A．when | B．while |
| C．after | D．since |
Tom was about to close the window his attention was caught by a bird.
| A．when | B．if |
| C．and | D．till |
—Could I use your computer for a few moments，please?
—________.I’m not using it myself.
| A．Come on | B．It depends |
| C．Go ahead | D．That’s great |
—Could I ask you a rather personal question?
—Sure，________.
| A．pardon me | B．go ahead |
| C．good idea | D．forget it |