A new study links heavy air pollution from coal burning to shorter lives in northern China.Researchers guess that the half-billion people alive there in the 1990s will live an average of 5 years less than the southerners because they breathed dirtier air.                China itself made the comparison possible for decades, a then government policy provided free coal for heating, but only in the colder north. Researchers found great differences in both air pollution and the length of life in the two areas. "This study provides a unique basis for the question about the life span in northern China because the heating policy dramatically changes pollution concentrations ," Michael Greenstone, a professor of environmental economics at Massachusetts Institute of Technology, 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 periods". The policy gave free coal for fuel boilers to heat homes and offices to cities north of the Huai River which divides China into north and south. It was in effect for much of the 1950-1980 period, of central planning,and though such policy was stopped after 1980, the practice of heavy coal burning in the north goes on, which releases heavy 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 particulates sent out from 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 the length of life using death data from 1991-2000.They found that in the north, the concentration of pollutants was 184 micrograms per cubic meter, 55 percent higher than in the south, and life length is 55 years lower on average. The main idea of this passage is that   _  .
PM2.5 is of especially great health concern Researchers find air pollution shortens life the government provided free coal for heating in North China coal burning causes bad air quality across China
Question: A new study links heavy air pollution from coal burning to shorter lives in northern China.Researchers guess that the half-billion people alive there in the 1990s will live an average of 5 years less than the southerners because they breathed dirtier air.                China itself made the comparison possible for decades, a then government policy provided free coal for heating, but only in the colder north. Researchers found great differences in both air pollution and the length of life in the two areas. "This study provides a unique basis for the question about the life span in northern China because the heating policy dramatically changes pollution concentrations ," Michael Greenstone, a professor of environmental economics at Massachusetts Institute of Technology, 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 periods". The policy gave free coal for fuel boilers to heat homes and offices to cities north of the Huai River which divides China into north and south. It was in effect for much of the 1950-1980 period, of central planning,and though such policy was stopped after 1980, the practice of heavy coal burning in the north goes on, which releases heavy 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 particulates sent out from 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 the length of life using death data from 1991-2000.They found that in the north, the concentration of pollutants was 184 micrograms per cubic meter, 55 percent higher than in the south, and life length is 55 years lower on average. The main idea of this passage is that   _  .
A. PM2.5 is of especially great health concern
B. Researchers find air pollution shortens life
C. the government provided free coal for heating in North China
D. coal burning causes bad air quality across China
Answer: