For a particle moving in a straight line, the displacement of the particle at time `t` is given by
`S=t^(3)-6t^(2) +3t+7`
What is the velocity of the particle when its acceleration is zero?
A. `- 9 ms^(-1)`
B. `-12 ms^(-1)`
C. `3 ms^(-1)`
D. `42 ms^(-1)`