题目内容：

A new study links heavy air pollution from coal burning to shorter lives in northern China. Researchers estimate that the half-billion people alive there in the 1990s will live an average of 5 years less than their southern counterparts because they breathed dirtier air.

    China itself made the comparison possible: for decades, a now-discontinued government policy provided free coal for heating, but only in the colder north. Researchers found significant differences in both particulate pollution of the air and life expectancy in the two regions.

    While previous studies have found that pollution affects human health, "the deeper and ultimately more important question is the impact on life expectancy," said one of the researchers, Michael Greenstone, a professor of environmental economics at Massachusetts Institute of 一Technology. "This study provides a unique setting to answer the life expectancy question because the(heating) policy dramatically changes pollution concentrations（含量），，，Greenstone said in an email. "Further, due to the low rates of migration in China in this period, we can know people's exposure over long time periods," he said.

    The policy gave free coal for fuel boilers to heat homes and offices to cities north of the Huai Riv饥which divides China into north and south. It was in effect for much of the 1950-198,0 period of central planning, and, though discontinued after 1980, it has left a legacy（遗留） in the north of heavy coal burning, which releases particulate pollutants into the air that can harm human health. Researchers found no other government policies that treated China's north differently from the south.

    The researchers collected data for 90 cities, from 1981 to 2000, on the annual daily average concentration of total suspended（悬浮的）particulates. In China, those are considered to be particulates that are 100 micrometers or less in diameter, sent out from sources including power stations, construction sites and vehicles. Among them, PM2.5 is of especially great health concern because it can go deep into the lungs.

    The researchers estimated the impact on life expectancies using death data from 1991-2000． They found that in the north, the concentration of particulates was 184 micrograms per cubic meter一or 55 percent higher than in the south, and life expectancies were 5.5 years lower on average across all age ranges.

1.The main idea of this passage is that＿·

A. research in China finds air pollution shortened life expectancy

B．the government provided free coal for heating in North China

C．coal burning causes bad air quality across China

D. a new study finds different particulates in South China

2.According to Greenstone,＿greatly contributed to the high pollution concentrations in North China.

A. power stations B. construction sites

C. the free heating policy D. gases from vehicles

3.It is implied in the passage that＿·

A. coal is no longer used for heating in North China

B．air quality was comparatively better in South China

C. southerners burned coals for heating in the 1980s

D．people preferred to live in South China after 1980

4.The underlined word "particulates" most probably means＿．

A. dirty clouds B. particular smoke

C. harmful dust D. dangerous bacteria