A group of young Americans have come to Beijing for a visit. Half of them, ___is, 50 persons, can speak Chinese fluently.
| A．that | B．which | C．who | D．it |
Only after a baby seal is pushed into the sea by its mother _____ to swim.
| A．how will it learn | B．will it learn how | C．it will learn how | D．and it learns how |
As is well known, carelessness and irresponsibility _______ many deadly accidents.
| A．lead to | B．result from | C．contribute to | D．result in |
They ___________ for two hours
| A．is working | B．worked | C．have been working | D．had worked |
Taiyuan，a city in North China，was struck by a heavy snowfall last week，________serious traffic problems.
| A．to face | B．faced | C．facing | D．having faced |
It was not until the beginning of the twentieth century________to explore space.
| A．did man begin | B．when man began |
| C．that man began | D．then man began |