Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

The air above our head is becoming cleaner. A breath of fresh air has been running right round the planet for the past five years. The planet is apparently purging itself of pollution. Paul Novell of the University of Colorado, the co-author of a report on this phenomenon says, “It seems as if the planet’s own cleaning service has suddenly got a new lease of life. Suddenly, there are a lot of changes going on up there”.

Estimates of the death toll from urban smog have been steadily rising, so the new cleaner trend could have significant consequences for life expectancy in cities as well as for the planet itself. The sudden and unexpected reversal of several decades of worsening pollution extends from the air in city streets to the remotest mid-Pacific Ocean and Antarctica.

Among the pollutants which have begun to disappear from the atmosphere are carbon monoxide, from car exhaust and burning rain forests, and methane from the guts of cattle, paddy fields, and gas fields. Even carbon dioxide, the main gas behind global warning, has fallen slightly.

There are two theories about why pollution is disappearing. First that there is less pollution to start with due to laws to cut down urban smogs and acid rain starting to have a global impact. Second, that the planet may be becoming more efficient at cleaning up.

The main planetary clean-up agent is a chemical called hydroxyl. It is present throughout the atmosphere in tiny quantities and removes most pollutants from the air by oxidizing them. The amount of hydroxyl in the air has fallen by a quarter in 1980s. Now it may be reviving for two reasons: ironically, because the ozone hole has expanded, letting in more ultraviolet radiation into the lower atmosphere, where it manufactures hydroxyl. Then the stricter controls on vehicle exhausts in America and Europe may have cut global carbon monoxide emissions, thereby allowing more hydroxyl to clean up other pollutants.

The word “it” in paragraph 5 refers to ______.