A block of mass $m$ is placed on a surface with a vertical cross-section given by $y=\frac{x^{3}}{6} .$ If the co-efficient of friction is $0.5$, the maximum height above the ground at which the block can be placed without slipping is:
(A) $\frac{1}{6} m$
(B) $\frac{2}{3} m$
(C) $\frac{1}{3} m$
(D) $\frac{1}{2} m$