Though plastic bags really bring convenience to us，they also ________ many problems for the environment.
| A．lead to |
| B．refer to |
| C．contribute to |
| D．appeal to |
The crazy fans ________ patiently for two hours and they would wait till the movie star arrived.
| A．were waiting | B．had been waiting | C．had waited | D．would wait |
He kept looking at her, wondering whether he ________ her somewhere.
| A．saw | B．has seen | C．sees | D．had seen |
She has set a new record, that is, the sales of her latest book ________ 50 million.
| A．have reached | B．has reached | C．are reaching | D．had reached |
—You were out when I dropped in at your house.
—Oh, I ________ for a friend from England at the airport.
| A．was waiting | B．had waited | C．am waiting | D．has waited |
It is said in the book that Thomas Edison (1847—1931) ________ the world-leading inventor for sixty years.
| A．would be | B．has been | C．had been | D．was |