[单项选择] More teenagers smoke today than at any 【B1】 since the 1970s, a federal study shows. Overall. 35% of children in grades 9-12 smoke cigarettes, a greater 【B2】 than 25% of adults who smoke. 【B3】 it is illegal to buy cigarettes until age 18. More than 75% of kids who 【B4】 them in stores are not asked for proof of 【B5】 , and they get cigarettes just as adults do: they walk in and buy them, 【B6】 no one checks their age. At a high school 【B7】 inaugurated a smoke-free campus this year, students who smoke cigarettes between classes 【B8】 that stricter laws will cut 【B9】 . Sixth and seventh-graders 【B10】 at a middle school say they have little trouble 【B11】 stores to sell them cigarettes, 【B12】 unsupervised vending machines or getting 【B13】 students to buy cigarettes for them. They say they have 【B14】 the most successful store-buying line early: "My 【B15】 is waiting in the car." 【B16】 smoking becomes less popular in adults, smoking becomes a form of rebelli
A. Since
B. Because
C. When
D. While