单选题 .  Doctors believe that second-hand smoke may cause lung cancer in people who do not smoke. Nonsmokers often breathe in the smoke from other people's cigarettes. This is second-hand smoke. The U.S. Environmental Protection Agency (美国环保局) reports that about fifty-three thousand people die in the United States each year as a result of exposure (暴露) to second-hand smoke.
    It is harder for children to avoid second-hand smoke. In the United States, nine million children under the age of five live in homes with at least one smoker. Research shows that these children are sick more often than children who live in homes where no one smokes. The damaging effects of second-hand smoke on children also continue as they grow up. The children of smokers are more than twice as likely to develop lung cancer when they are adults as children of nonsmokers. The risk is even higher for children who live in homes with both parents smoke.
    People are becoming very aware of the danger of second-hand smoke. As a result, they have passed laws that prohibit people from smoking in many public places. Currently, 45 states in the United States have laws that restrict, or limit smoking. The most well-known law doesn't allow people to smoke on short native airline flights, i.e. flights within the country.1.  Compared with nonsmokers' children, the children whose parents both smoke are likely to ______.