题目

Good afternoon, everyone,     I'd like to introduce my friend Christy to you all. Christy is a great example of how one person with  36  can make her dream a reality, so she should win the Student of the Year Award.     Christy loves to ride bikes. She does not just ride along city streets,   37  . She is interested in  38   trail (小路) rides.  39  Christy knew that people who love riding had to travel  40  the city to locate good bike trails, so she  41  to do something.     Christy knew that Roosevelt Park had a clean, lively creek(小溪)  42  through it.关于做匀速圆周运动的物体所受的向心力，下列说法中正确的是（　　）A．物体除其他的力外还要受到一个向心力的作用B．物体所受的合外力提供向心力C．向心力是一个恒力D．向心力是根据性质命名的一种力