正确答案:

题目：燃放烟花爆竹产生的物质会对空气造成污染。 （ ）

解析：√

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举一反三的答案和解析：

[单选题]AIn China，more and more middle school students are getting shorter sleeping time than be-fore． Most students sleep less than nine hours every night，because they have much homework to do. Some homework is given by their teachers，and some by their parents． Also，some students don't know how to save time .They are not careful enough while they do their homework，so it takes them a Iot of time． Some students spend too much time watching TV or playing computer games．They stay up very late． Some students have to get up early every morning on weekdays to get to school on time by bus or by bike．lt can be a long way from home to school．

Doing their homework．

解析：21．C【解析】由第一段第二句可知由于做作业，大多数学生睡眠少于9小时。

[单选题]20．

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