Câu hỏi:

Read the text carefully, then choose the most suitable answers.

Smog is a combination of smoke and fog It occurs when gases from burnt fuel mix with fog on the ground. When heat and sunlight get together with these gases, they form fine , dangerous particles in the air.

Smog occurs in big cities with a lot of traffic. Particularly, in the summertime, when it is very hot, smog stays near the ground. It is dangerous to our breathing, and in smoggy areas, we cannot see very well.

Smog was first discovered in Great Britain in the 19th century, during the beginning of the Industrial Revolution. At that time, people used coal for heating and cooking. Factories also used coal to produce iron and steel. Smoke mixed with wet, foggy air and turned yellow. The smog often hung over cities for many days. It caused lung diseases and breathing problem. Thousands of people died in London every year.

Today, cities that have a large population and are located in hot, subtropical areas have the biggest smog problems - for example, Los Angeles, Mexico City and Cairo.

 In the third paragraph, the word it refers to ______.