If a body moves in a straight line according to the law s = 24t + 3t^2 - t^3, where s is the distance measured in meters from the origin and t is the time in seconds after it starts to move, calculate the body's velocity as a function of time.
A. 63 m/s
B. 15 m/s
C. 27 m/s
D. 81 m/s