In our daily life, some people think that smoking1help them relax. In fact, smoking is really2 Do you know that there are over 4,000 chemicals（化学药品） in cigarettes? These chemicals are 3for our bodies. They can cause cancer and 4diseases.

    The smoke from cigarettes harms not only smokers but also other people. It's5“second-hand smoke” and it can be6and has higher risks. Smokers shouldn't use smoking7them relax. They must give up smoking as soon as possible.

曲线y=$\frac{2}{x}$与直线y=x﹣1及x=4所围成的封闭图形的面积为（　　）

用右图的装置来测定空气中氧气的体积分数。