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