The position of an object moving in one dimension is given by x(t) = t^3 - 15t^2 + 72t. What is the earliest time at which the object is momentarily at rest?

a) 1.00 s b) 2.00 s c) 3.00 s d) 4.00 s e) 5.00 s

the answer is D. Can you explain why