Test and analysis: in the process of uniform circular motion, although the motion speed has not changed, the motion direction has changed, so the motion state has changed, indicating that the object must be stressed and the force must be unbalanced, so B is selected.

Comments: The key to solve this problem is to be familiar with the relationship between force and motion, and to understand the reasons why history changes the motion state of an object. Force is not the reason to maintain the state of motion of an object.