单项选择题

Questions 14 to 17 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.
What gas does not pollute the air

A．Oxygen.
B．Hydrocarbons.
C．Carbon monoxide.
D．Nitrogen oxides.

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单项选择题

单项选择题

Questions 14 to 17 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. What gas does not pollute the air 您的浏览器不支持 audio 元素。