# A solid sphere of mass m is lying at rest on a rough horizontal surface. The coefficient of friction between the ground and sphere is $\mu$. The maximum value of F, so that the sphere will not slip is equal to
$\begin {array} {1 1} (a)\;\frac{7}{5} \mu mg & \quad (b)\;\frac{4}{7} \mu mg \\ (c)\;\frac{5}{7} \mu mg & \quad (d)\;\frac{7}{2} \mu mg \end {array}$