The Equation Describing the Position of an Object Undergoing Simple $x = ( 5.0 \mathrm {~cm} ) \cos ( 10 \pi t - \pi / 2 )$

The equation describing the position of an object undergoing simple harmonic motion is $x = ( 5.0 \mathrm {~cm} ) \cos ( 10 \pi t - \pi / 2 )$ , where t is in seconds. The angular frequency of the motion is

A) 5 Hz.
B) 5 rads.
C) 10 $\pi$ Hz.
D) 10 $\pi$ rads.