500 years ago in Britain, the burning of coal (煤) 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 (工业革命) 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 (烟雾). 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 (法律) 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 switch to other fuels such as gas. Factories started using tall chimneys (烟囱) 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.

77. From the passage, we learn that smog was a big problem in ______ in London.

A. spring　　　　　　 B. summer　　　　　 C. autumn　　　　　 D. winter

78. The underlined words "switch to" mean "______" in Chinese.

A. 发明　　　　 B. 换用　　　　 C. 拒绝　　　　 D. 节省

79. Which was NOT a way people in Britain used to make the air cleaner, according to the passage?

A. Factories built higher chimneys.

B. People began to use less coal.

C. More People began to ride bikes.

D. New factories were built outside cities.

80. Which is the best title for the passage?

A. Air Pollution in Britain in the Past.

B. How to Make the Air Cleaner.

C. Smog Is Bad for the Environment.

D. New Laws to Stop Air Pollution.

What do you do if you're stranded（处于困境的）on a lonely island? Surely you need to find a way to get in touch with the outside world. Your best chance of doing this is to draw the attention of a passing plane.
　　Body signals（信号）
　　Pilots from different countries understand body signals if they have the correct training. When you wave your arms up and down in a straight line, it means "yes". When you point downwards and swing your arm from side to side, it means "no". If you want the pilot to know that it's safe to land, push your hands out in front of you and bend（弯曲）your knees. If you want to say that it's not safe to land, put your arms in the air and move them to one side.
　　Smoke signals
　　The smoke from a fire can be seen from far away, so it's a good way to draw attention. If the weather is dry, it isn't hard to start a fire. Remember, however, that fires can be very dangerous if they get out of control. Never light a fire unless you're sure that it can't spread. If the ground is dark, light smoke can be seen more easily. Green grass and leaves produce light smoke.
　　Ground-to-air signals
　　It's a good idea to build some signals, too. Use large pieces of wood to make the symbols(标记). If you can't find any wood, use earth. Some useful symbols are:
　　F "I need food and water."
　　II "I need medicine."
　　I "I am badly hurt."
　　X" I am unable to move from here."
　　Pilots' replies
　　If the pilot lowers the plane's wings from side to side, this means "message received and understood". (At night, the pilot flashes the plane's green lights.) If the pilots flies the plane in a clockwise circle, this means "message received but not understood".(At night, the pilot flashes the plane's red lights.)
However, there is no signal which means "message not received". Why not?
【小题1】 Which picture means "it's not safe land"?

　
【小题2】 What can we learn from the passage?