根据短文理解，从A、B、C三个选项中选择最佳答案填空。
People usually check whether their keys, purses and phones are with them before going out, but many people in China now have one more important thing to take--a mask(口罩). They hope it can protect them 1 terrible air.
Many cities in China saw a lot of foggy days in April. In Beijing, there were eight days in April full of smog. It is dirty air 2 looks like a mixture of fog and smoke. The smoke is mainly from cars and factories in cities. Poor air is bad for people's health, 3 for children and old people. They fell ill easily and the 4 were full of people. They had problems with their noses and lungs.
So it's necessary 5 people from doing so many bad things to the environment. For example, people should use cars less and 6 burn lots of coal and gas. Government needs to take measures to make factories stop pouring 7 waste air into the sky.
Three students from No. 4 High School in Beijing 8 a report to advise how to protect the environment. They suggested 9people should plant more trees and act off fewer fireworks for celebration. We hope more people will join us in 10 a difference. In this way, the blue sky will certainly return for good.
