单选题

　　[填句补文]Doctors believe that secondhand smoke may cause lung cancer in people who do not smoke because nonsmokers often breathe in the smoke from other people‘s cigarettes.________. The US Environmental Protection Agency reports that about 53，000 people die in the United States each year as a result of exposure to secondhand smoke.

　　A、The risk is even higher for children who live in homes where both parents smoke.

　　B、In the United States，nine million children under the age of five live in homes with at least one smoker.

　　C、This smoke is called secondhand smoke.

　　D、No one knows for sure why Mr. Gilson has lung cancer.