单选题
[填句补文]The smoke that comes from a lit cigarette contains many different poisonous chemicals. In the past， scientists did not think that these chemicals could harm a nonsmoker's health. ________. They discovered that even nonsmokers had unhealthy amounts of these toxic （有毒的） chemicals in their bodies.
A.The Gilsons have been married for 35 years.
B.Recently， though， scientists changed their opinion after they studied a large group of nonsmokers.
C.We cannot avoid secondhand smoke in restaurants.
D.In the United States， nine million children under the age of five live in homes with at least one smoker.
正确答案：B
答案解析：前一句讲的是科学家们过去对二手烟的看法，接下来应选表示他们改变观点的句子，因为下一句表明他们所持的观点已与过去截然不同。