【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.
