These problems，______ will seriously affect the growth of teenagers and even the nation's future.
| A．not if solved properly |
| B．if solved properly |
| C．not if properly solved |
| D．if not solved properly |
— What made your skin look so terrible?
— _______ to the sunlight for a long time.
| A．Because it was exposed | B．After being exposed | C．Being exposed | D．Exposed |
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 |