参考答案

正确答案：D
解析：推理题。文中提到The study also showed that the danger of lung cancer increased only for sons and not for daughters，and that the father's smoking did not affect a child's chance of developing lung cancer．父亲吸烟不会对孩子造成影响，母亲吸烟只对儿子造成影响。根据这一研究结果可以推出正确答案为选项D。