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  3. Hydrocarbons, ______ by engine exhausts, react with nitrogen oxides in the presence of sunlight to form complex toxic gases.

Hydrocarbons, ______ by engine exhausts, react with nitrogen oxides in the presence of sunlight to form complex toxic gases.

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问题 Hydrocarbons, ______ by engine exhausts, react with nitrogen oxides in the presence of sunlight to form complex toxic gases.
选项 A、are given off
B、give off
C、they are given off
D、given off
答案D
解析 本句的谓语为后面的react，空格处应用分词的形式，意为“机车排出的废气”。又由于give off与主语hydrocarbons是被动关系，所以用过去分词。因此D项正确。
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考研英语一

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