If a body of mass $m$ is moving on a rough horizontal surface of coefficient of kinetic friction $\mu$, the net electromagnetic force exerted by surface on the body 18
(a) $m g \sqrt{1+\mu^{2}}$
(b) $\mu \mathrm{mg}$
(c) $\mathrm{mg}$
(d) $m g \sqrt{1-\mu^{2}}$