A Mass on a Spring Vibrates Horizontally on a Smooth $x(t)=9 \sin t$

Multiple Choice

A mass on a spring vibrates horizontally on a smooth level surface (see the figure) . Its equation of motion is $x(t) =9 \sin t$ , where t is in seconds and x in centimeters. Find the velocity at time t.

A) $v(t) =9 \cos t$
B) $v(t) =2 \cos 9 t$
C) $v(t) =\cos 9 t$
D) $v(t) =9 \sin 9 t$
E) $v(t) =\sin 9 t$