2. 外语
  3. There are a number of organizations and books that can help you learn more about ______indoor air quality.

There are a number of organizations and books that can help you learn more about ______indoor air quality.

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问题 There are a number of organizations and books that can help you learn more about ______indoor air quality.
选项 A、to improve
B、how to improve
C、improve the
D、how improve
答案B
解析 The infinitive phrase how to improve is the object of the preposition about.
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