Section B
Directions: There are 2 passages in this section．Each passage is followed by some questions or unfinished statements．For each of them there are four choices marked A, B, C and D．You should decide on the best choice.
Scientists in the United States have reported some good news about the ozone in the atmosphere．Recent findings suggest that the destruction of ozone by pollution is slowing．Ozone is a form．of oxygen．Its presence above earth protects us from radiation from the sun.
In one study, researchers examined information gathered by space agency satellites and by equipment on earth．The findings are, in the words of Michael Newchurch of the University of Alabama: 'This is the beginning of a recovery of the ozone layer.' Mr．Newchurch led the study．He says the atmosphere was losing about 8 percent of the ozone layer per 10-yeas period since the late 1970s．In the last 5 years, though, that rate of loss has dropped by half．He says the atmosphere should start to gain ozone before long．He says a full recovery, however, is about 50 years away, as long as the Montreal Protocol remains in place.
That is an international treaty from 1987 to restore the ozone layer．The treaty restricts the use of a number of chemicals that destroy ozone, like chlorofluorocarbons(氯氟碳化合物), or CFC.
Wide use of CFC began in the 1930s, They became popular coolants in devices such as refrigerators and air conditioners．CFC remains in the atmosphere fox years.
Mr．Newchurch says the study provides evidence that the Montreal Protocol is working by reducing CFC pollution, But the study found the ozone improvement only in the upper stratosphere(平流层)．The scientists have not yet seen evidence of similar changes in the lower stratosphere．That holds most of the protective ozone.
The Montreal Protocol also restricts the use of methyl bromide(甲基溴), a chemical to kill insects．Farmers and shippers are the main users．Natural sources of methyl bromide include oceans and some plants．Scientists say the bromine gas it produces is 50 times more destructive to ozone than chlorine(氯气)gas from CFC.
Methyl bromide has never been used as widely as CFC．But a separate study found a decrease in bromine gas levels in the atmosphere．Researchers at the National Oceanic and Atmospheric Administration did the study．They reported a 13 percent drop since 1998．The report is based on 8 years of measurements taken at 10 stations around the world.
American scientists are very concerned about the ozone because ______.