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PASSAGE 28

Pollutants in the air aren’t always visible and come from many different sources. Smog hanging over

cities is the most familiar and obvious form of air pollution. But there are different kinds of pollution –

some visible, some invisible – that contribute to global warning. Generally any substance that people

introduce into the atmosphere that has damaging effects on living things and the environment is

considered air pollution.

Carbon dioxide, a greenhouse gas, is the main pollutant that is warming Earth. Though living things

emit carbon dioxide when they breathe, carbon dioxide is widely considered to be a pollutant when

associated with cars, planes, power plants, and other human activities that involve the burning of fossil

fuels such as gasoline and natural gas. In the past 150 years, such activities have pumped enough carbon

dioxide into the atmosphere to raise its levels higher than they have been for hundreds of thousands of

years.

Other greenhouse gases include methane – which comes from such sources as swamps and gas emitted

by livestock – and chlorofluorocarbons (CFCs), which were used in refrigerants and aerosol propellants

until they were banned because of their deteriorating effect on Earth's ozone layer.

Another pollutant associated with climate change is sulfur dioxide, a component of smog. Sulfur

dioxide and closely related chemicals are known primarily as a cause of acid rain. But they also reflect

light when released in the atmosphere, which keeps sunlight out and causes Earth to cool. Volcanic

eruptions can spew massive amounts of sulfur dioxide into the atmosphere, sometimes causing cooling

that lasts for years. In fact, volcanoes used to be the main sources of atmospheric sulfur dioxide; today

people are.

Industrialized countries have worked to reduce levels of sulfur dioxide, smog, and smoke in order to

improve people’s health. But a result, not predicted until recently, is that the lower sulfur dioxide levels

may actually make global warming worse. Just as sulfur dioxide from volcanoes can cool the planet by

blocking sunlight, cutting the amount of the compound in the atmosphere lets more sunlight through,

warming the Earth. This effect is exaggerated when elevated levels of other greenhouse gases in the

atmosphere trap the additional heat.

Question 1. According to the passage, some kind of air pollution

.

A. are not introduced into the atmosphere

B. do not damage the environment

C. cannot be seen with our naked eyes

D. do not affect living things seriously

Question 2. What is the main origin of carbon dioxide in the air?

A. Living things' breathing

B. Manufacturing cars

C. Building power plants

D. Burning coals and petrol

Question 3. What were banned because of their damaging the ozone layer?

A. Chlorofluorocarbons

B. Methane and gas

C. Refrigerants

D. Aerosol propellants

Question 4. What is NOT mentioned as a feature of sulfur dioxide?

A. Creating smog

B. Causing acid rain

C. Reflecting light

D. Warming the Earth up

Question 5. What has been the main source of atmospheric sulfur dioxide recently?

A. Human activities

B. Volcanic eruptions

C. Forest fires

D. Earthquakes

Question 6. What does the word “the compound” in the last paragraph refer to?