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The air is usually dirty in winter. Smog (雾霾) makes the air dirty. We can find smog in many cities. Smog becomes a big problem in China. We cannot see clearly in the heavy smog. Sometimes, we cannot drive either. Smog is harmful (有害的). It makes people ill. Old people have to stay at home. Children cannot play in the playground. They have to stay in their classrooms. Sometimes they cannot go to school. Where does smog come from? Many people think smog comes from coal. In the winter, factories and homes use lots of coal. Some people think smog comes from cars. What should we do if (如果) there is smog? We should stay at home and not go outside (外出).
1．We cannot find smog in spring, summer or winter.2．There is smog in many cities in China.
3．Smog is good because children do not have to go to school.
4．Old people should go outside in the smog.
5．Smog does not come from cars.
同类型试题
y = sin x， x∈R， y∈[–1，1]，周期为2π，函数图像以 x = (π/2) + kπ 为对称轴
y = arcsin x， x∈[–1，1]， y∈[–π/2，π/2]
sin x = 0 ←→ arcsin x = 0
sin x = 1/2 ←→ arcsin x = π/6
sin x = √2/2 ←→ arcsin x = π/4
sin x = 1 ←→ arcsin x = π/2
y = sin x， x∈R， y∈[–1，1]，周期为2π，函数图像以 x = (π/2) + kπ 为对称轴
y = arcsin x， x∈[–1，1]， y∈[–π/2，π/2]
sin x = 0 ←→ arcsin x = 0
sin x = 1/2 ←→ arcsin x = π/6
sin x = √2/2 ←→ arcsin x = π/4
sin x = 1 ←→ arcsin x = π/2
