________ tonuclear radiation, even for a short time, may produce variants（变异体）of genes in human bodies.A.BeingexposedB.HavingexposedC.ExposedD.Afterbeing exposed
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A解析注意到这道题中有一个插入语“ever for a short time”，可先去掉以便分析句子，插入语前面的是主语，必须用非谓语动词，再根据句意可知必须用被动语态。