A block of mass m is placed on a rough horizontal surface. The coefficent of friction between them is `mu`. An external horizontal force is applied to the block and its magnitude is gradually increased. The force exerted by the block on the surface is R.
A. The magnitude of R gradually increase.
B. `R le mg sqrt(mu^(2)+1)`.
C. The angle made by R with the vertical will gradually increase.
D. The angle made by R with the vertical `le tan^(-1)mu`.
