A massless string is wrapped around a hollow cylinder having mass `m` and radius `r`. The cylinder is kept on a rough horizontal surface (coefficient of friction is `mu`). A constant force `F` is applied as shown in the figure. In case of pure rolling, the friction force acting on the bottom most point of the cylinder is
A. zero
B. `mu mg`
C. `mu mg//2`
D. `3mu mg//2`