A block of mass m is placed on a surface with a vertical cross section given by $y = x^3 /6$. If the coefficient of friction is 0.5, the maximum height above the ground at which the block can be placed without slipping is :

Question

( A ) $\frac{1}{3} m$
( B ) $\frac{1}{6} m$
( C ) $\frac{2}{3} m$
( D ) $\frac{1}{2} m$