500 years ago in Britain, the burning of coal   was increasing in cities like London. Coal was used in factories and also used to heat homes. Coal, when burnt makes a lot of smoke, which makes the air very dirty. About 200 years ago, the Industrial Revolution   began in Britain. Factories were built, and even more coal was burnt. Air pollution was becoming a really big problem. Smoke and fog together make smog  . Smog was a big problem in London's winter. Because of the cold weather, more coal was burnt to warm houses and this made more smoke. When smog stayed over a city, it became really hard to breathe and see clearly. In 1952, the Great London Smog happened and more than 4,000 people died because of the smog. New laws   were made in 1956 and 1968 to stop such sad things from happening again. These laws were called the Clean Air Acts. These laws were made so that air would become cleaner. The laws encouraged people to use less coal or _ other fuels such as gas. Factories started using tall chimneys   so that the smoke would go high up in the sky and no longer cover cities, and new factories were built in the countryside. Smog appeared less often and the air became cleaner. From the passage, we learn that smog was a big problem in   _   in London.
spring summer autumn winter