单项选择题

Questions 17 to 20 are based on the following passage. At the end of the passage, you will be given 20 seconds to answer the questions. Now, listen to the passage. Air pollution refers to the presence of all the following EXCEPT______.

A．harmful solids
B．harmful gases
C．harmful liquids
D．harmful chemicals

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单项选择题

单项选择题

Questions 17 to 20 are based on the following passage. At the end of the passage, you will be given 20 seconds to answer the questions. Now, listen to the passage. Air pollution refers to the presence of all the following EXCEPT______. 您的浏览器不支持 audio 元素。