A particle is moving in a straight line. The variation of position $x$ as a function of time $t$ is given as $x = \left( t ^ { 3 } - 6 t ^ { 2 } + 20 t + 15 \right) \mathrm { m }$. The velocity of the body when its acceleration becomes zero is:\\
(1) $4 \mathrm {~m} \mathrm {~s} ^ { - 1 }$\\
(2) $8 \mathrm {~m} \mathrm {~s} ^ { - 1 }$\\
(3) $10 \mathrm {~m} \mathrm {~s} ^ { - 1 }$\\
(4) $6 \mathrm {~m} \mathrm {~s} ^ { - 1 }$