题目

The World Health Organization says air pollution is now the world's single-largest environmental health risk. A new report from the WHO says nearly eight million people died in 2012 because of air pollution. That is more than twice as many as the number in 2008. Air pollution is an international problem. The World Health Organization says one in eight people die from air pollution, from both inside buildings and outside. It says new information shows air pollution is partly to blame for people dying at an early age from h eart disease. It says other causes of death include canc在样本的频率分布直方图中，共有9个小长方形，已知中间一个长方形的面积等于其他8个长方形面积的和的．又样本容量是100，则中间这一组的频数是________．