2. 外语
  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 give off释放；此句的谓语为后面的react，所以此处应用分词的形式。又由于give off与主语Hydrocarbons是被动关系，所以用过去分词。
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