London used to be"foggy"---the"fog"was in fact smog，amixture of smoke and fog．In other words，it was caused by air pollution．In London，some of this pollution came from factories，but a lot of it came from the coal that people burnt in their houses to keep warm during the winter．By the 1950s，London's smog problem had become so bad that the government decided to do something to clean up the air．It didn't allow people to born coal in any British city．A few years later，the air became much cleaner．
Today many Chinese cities face the same problem with air pollution that London faced sixty years ago．However，this problem is more difficult for Chinese cities to solve．One reason is that more of pollution comes from big factories，rather than from the coal burntin people's houses．If these factories were suddenly closed，it would harm the economy and lots of people would lose their jobs．Another reason is that changing from coal to cleaner fuels is quite expensive．However，the air in many cities will become cleaner，as the government and ordinary people pay more and more attention to protecting the environment．
38．Where was the pollution from in London over sixty years ago？___
A．The pollution came from factories．
B．The pollution was from coal that people burnt in their houses．
C．The pollution was from too much traffic on the roads．
D．The pollution was from factories and coal that people burnt in their houses．
39．How was the air in London after the government didn't allow people to born coal in any British city？___
A．It was still foggy．
B．It was smog．
C．It was much cleaner．
D．It was stormy．
40．What is the main idea of the second paragraph？___
A．In most of Chinese cities there is smog．
B．The reasons of smog in China．
C．The reasons of why it's difficult for China to solve"smog"．
D．What did Chinese government do