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)_做题室

单项选择题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：
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