London used to be "foggy"--- the "fog" was in fact smog, a _ 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 burnt in 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. Which of the following is not true EXCEPT   _  .
people in London were allowed to born coal sixty years ago. the air pollution in many Chinese cities is not serious. smog in fact is a mixture of smoke and fog. changing from coal to cleaner fuels is quite easy and cheap.