Expert have been warning-----of the health risks caused by p

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Expert have been warning-----of the health risks caused by passive smoking.
A.at a time B.at one time C.for some time D.for the time 应该选什么,为什么,它们各是什么意思?

应该选B,整句意思是,专家曾经警告我们,吸二手烟会导至健康风险.
A.at a time 每次,在某时,B.at one time 同时,曾经 C.for some time 一段时间 D.for the time being 暂时

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