4. When SARS ________, fear _______ quickly throughout the country.

3. Whether the environment will improve ________ the efforts of the government and the companies.

2. No one believes that ghosts ________ in our life.

explore, allow, develop, spread, depend on, prevent, exist，break out

1. The American pilot is believed to have ________ outer space last year.

8. They stood still, looking at the balloons_________ (漂浮) in the air.

7. His health _______(逐渐地) improved.

6. _________ (全球的) warming has become a big problem.

5. The dogs can be trained to detect the _________ (存在) of explosives.

4. With the _________(发展) of modern society, many new techniques have been developed.

3. Many kinds of _________ (方法) of payment have been used in daily life.