专题限时训练(十八) [说明文型完形填空]
(限时：每篇10分钟)
(一)
A survey done by the University of Michigan shows a big drop in the numbe r of t eenagers who say they smoke cigarettes.
In the survey, over 44,000 students in grades 8, 10 and 12, from 424 schools across the __1__ were asked about their smoking habits. The results show that smoking among 8th graders __2__ from 21 percent in 2002, to 12 percent in 2007.In the same period, smoking dropped from 30% to 21 % among 10th graders and from 37 % to 30 % among 12th gra ders.
