It is a long time ________ they separated, but it won’t be long _____ they meet again.

　 A. since; before　 　　　　 B. after; when

C. since; when　　 　　　　　 D. after; before

Damage to Ozone layer Gets Worse

　 In the middle of winter, when snow is falling in many parts of the United States, scientists have sounded a warning to people who plan to spend many hours in the sun this summer. The warning is: The sun’s summertime rays are more dangerous than once thought.

　 A team of scientists from 80 nations recently reported to the United Nations that a layer of ozone (臭氧层) in the atmosphere, which protects humans from harmful levels of ultraviolet（紫外线） radiation, will be thinner over the United States this summer. The thinner layer allows more ultraviolet rays from the sun to reach Earth. The extra amount of ultraviolet radiation could cause an increase in the number of cases of skin cancer.

　 Scientists first became concerned about the ozone layer in the mid-1980s when a hole was discovered in the layer above Antarctica during the winter. The hole was caused by chemicals used in refrigerators and air conditioners. When these chemicals are sent out into the atmosphere, they produce gases that destroy the ozone.

　 Concern about the protective ozone layer rose more recently when data(数据) from satellites and ground stations showed that ozone levels were dropping over areas other than Antarctica. Low ozone levels were recorded in the spring and summer over the United States and over other populated areas in the world.

　 Although many countries have already begun stopping the use of ozone destroying chemicals, the new findings are expected to advance the timetable for a total ban(禁止) of the chemicals.

1.The scientists have observed the ozone layer_____.

A. since 1980　　　　　　 B. since last winter　　　

C for about 20 years　　　 D. for about one year

2. The ozone layer in the atmosphere can _____.

A.    do a lot of good to human beings in many ways.

B.    protect humans from diseases caused by bad weather.

C.    do a lot of harm to human beings in the summer-time.

D.    protect humans from harmful levels of ultraviolet radiation.

3. Scientists first found that there was a hole in the ozone layer_____.

A.       above Arctic during the winter in the mid-1980s

B.       above Antarctica during the winter in the mid-1980s

C.       over somewhere in the north of the equator in 1980

D.       over the U.S.A in the summertime in the mid-1980s

4.The damage to the ozone layer was caused by_____.

A.       the changeable weather

B.       ozone-destroying chemicals

C.       chemicals from refrigerators

D.       chemicals from air conditioners