An object of mass m is traveling at constant speed v in acircular path of radius r. How much work is done by the centripetal force during one-half of a revolution? 
选项为:A:$\pi mv^{2}$  B:$$0$$  C:$\pi mv^{2}r$  D:$2\pi mv^{2}r$  请回答其中一个选项.