An uniform thin rod OA is pivoted on a frictionless hinge at one end O, as shown in the figure. The rod is held at rest horizontally and then released. When it reaches the vertical position which one is correct of following statements ( )
A.
Angular velocity varies from small to big, angular acceleration remains unchanged;
B.
Bothe angular velocity and angular acceleration vary from small to big;
C.
Angular velocity varies from small to big, while angular acceleration varies from big to small;
D.
Angular velocity remains unchanged , angular acceleration equals zero.