The contact force exerted by a body A on another body B is equal to the normal force between the bodies. We conclude that
(a) the surfaces must be frictionless
(b) the force of friction between the bodies is zero
(c ) the magnitude of normal force equals that of friction
(d) the bodies may be rough but they don't slip on each other.

Options

A. a and c are true
B. a, b and d are true
C. b and d are true
D. all are true

Answer

C