The environment of this city will be more seriously polluted (污染) ______ effective measures are taken to prevent factories from sending out poisonous gases and wastewater. ( 章节：5 难度：3)
- A、if
- B、once
- C、however
- D、unless
正确答案:D
答案解析:有
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The environment of this city will be more seriously polluted (污染) ______ effective measures are taken to prevent factories from sending out poisonous gases and wastewater. ( 章节：5 难度：3)
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