If $s=r^{3}-4t^{2}+7,$ the velocity when the acceleration is zero is

Question

$$\begin{array}{1 1}(1)\frac{32}{3}m/sec&(2)\frac{-16}{3}m/sec\\(3)\frac{16}{3}m/sec&(4)\frac{-32}{3}m/sec\end{array}$$