题目内容：

    Breathing dirty air comes at a high price. Air pollution lowers the average life spans (寿命) by a year worldwide and in more polluted parts of Asia and Africa, dirty air shortens lives up to twice that much. Scientists shared their new findings in Environmental Science & Technology Letters. The study used data gathered in 2016 as part of a project known as the Global Burden of Disease and was the first major country-by-country look at the connection between the length of life and what’s known as fine PM.

Air pollution has been linked to many health problems. Most earlier studies had looked at how tiny air pollutants affected rates of illness or death. Joshua Apte is an environmental scientist at the University of Texas at Austin. By looking at life expectancy (预期寿命), his team had hoped to make the threat easier to understand. PM2.5 is what scientists call tiny particles (颗粒) of pollution in the air. Higher levels of PM2.5 can cause health problems and cut months, if not years, from the average length of life. This analysis shows how pollution affects life expectancy in different parts of the world.

The World Health Organization (WHO) recommends limiting PM2.5 to 10 micrograms per cubic meter of air. Apte’s group calculated how holding pollution to this low level would help people. In countries with very dirty air, meeting this standard would lengthen people’s lives. However, in countries whose air already meets this standard, the study shows no gain in life expectancy. In other words, meeting the WHO standard won’t reduce health costs resulting from dirty air because even below 10 micrograms per cubic meter, pollution still causes serious risks. Meanwhile, the scientists compared how other threats including smoking and cancer shorten the length of life across the globe.

1.What is special about the study?

A.It won recognition from a professional journal.

B.It discussed health problems caused by air pollution.

C.It gathered lots of data for the Global Burden of Disease.

D.It analyzed the link between life spans and PM by country.

2.What is Joshua Apte’s team trying to do?

A.Help people better understand air pollution.

B.Study life expectancy in different countries.

C.Know how small air pollutants affect health.

D.Deal with different kinds of health problems.

3.What will happen if a country limits PM2.5 to 10 micrograms per cubic meter of air?

A.People’s life spans will surely increase. B.It will get strong support from the WHO.

C.People’s health may not be much improved. D.It will guarantee people clean air.

4.What might be discussed in the following paragraphs?

A.How cleaning up the air can lengthen lives.

B.How air pollution shortens lives by country.

C.How other threats shorten life expectancy.

D.How all the countries deal with severe pollution.