Fighting an artificial greenhouse effect seems to be the greatest of environmental challenges, ________？
| A．isn't it | B．does it |
| C．doesn't it | D．is it |
21. Don’t pour water on his plan. We should encourage him.
| A．hot | B．cool | C．warm | D．Cold |
20. Don’t be too wordy. Just give me a very _____ answer.
| A．clean | B．clear | C．neat | D．Tidy |
19. The price of the car “Honda” is very _____ and my parents can’t afford it.
| A．cheap | B．high | C．expensive | D．Dear |
18. It is ______ that he has passed the examination, and I am _____ I will pass it too.
| A．certain; certain | B．certain; sure |
| C．sure; certainly | D．certainly; surely |
17. Be when you cross the road.
| A．careless | B．beware | C．careful | D．Carelessness |