_______the thick clouds, you would find the kite in the sky easily.
| A．Had it not been for | B．If it were not |
| C．If it had not been for | D．Were it not for |
They kept trying _____ they must have known it was hopeless.
| A．if | B．because | C．when | D．where |
An awful accident, however, _____occur the other day.
| A．does | B．did | C．has to | D．had to |
Little Jim is going to spend his holiday in Paris, _____ lives his uncle.
| A．which | B．who | C．where | D．that |
The person we referred to（提及）____ us a report tomorrow．
| A．giving | B．will give | C．gave | D．give |
The research is so designed that once _____ nothing can be done to change it.
| A．begins | B．having begun | C．beginning | D．begun |