单项选择题
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
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