The position of a toy locomotive moving on a straight track along the x axis is given by the equation x = t^3 - 6t^2 + 9t, where x is in meters and t is in seconds. The net force on the locomotive is equal to zero when t is equal to
(A) zero
(B) 2 s
(C) 3 s
(D) 4 s
(E) 5 s