题目

Group exercise is one of the most effective ways to improve physical fitness and sustain(保持) a healthy lifestyle. Group exercise is challenging, yet fun and empowering! Of course everyone knows that exercise is good for the body. However, studies have shown that when exercise is performed in groups, it's not only great for improving physical health but for psychological health. It's an opportunity to be social, release endorphins（内啡肽）, and improve your strength. Additionally, group exercise creates a community feel and the shared common goal motivates participants to work如图所示，原点O为三同心圆的圆心，大圆直径AB=4cm，则图中阴影部分的面积为（ ）A．4cm2B．1cm2C．4πcm2D．πcm2