When it______ air pollution, Beijing faces the three “C” s-----cars，coal and construction，which lead to Beijing's thick air pollution.
| A．refers to | B．comes to | C．happens to | D．speaks to |
To their disappointment, they found the lecture given by the professor difficult__________.
| A．to be understood | B．to understand |
| C．understanding | D．understood |
As soon as he heard the ________ news of his death, she _______ tears.
| A．shocked; burst out | B．shocking; burst into |
| C．shocked; burst into | D．shocking; burst out |
----I’m going to the United States.
---- How long ____you ____in the States ？
| A．are; stayed | B．are; staying |
| C．have; stayed | D．did; stay |
He often helps the students _________he thinks are not quick at their studies.
| A．whom | B．whose | C．when | D．who |
This painting of his had been ____________ by some experts of the world.
| A．high thought of | B．well thought of |
| C．highly thought | D．good thought |