Questions26-30are based on the following chart : 　　Cigarette Smoking by Grade Level: 2005-2009
 
　　(说明：2005年为白框，2007年为灰框，2009年是黑框，扫描问题无法显示。)

The purpose of the chart is to show（）

A．the percentages of smokers in schools
B．the increase of smokers in grades 9-12 during 2005-2009
C．the number of teenage smokers in grades 9-12
D．the amount of money spent by young smokers- over the years