Which of the following statements regarding laws governing the friction between dry surfaces are correct?
1. The friction force is dependent on the velocity of sliding.
2. The friction force is directly proportional to the normal force.
3. The friction force is dependent on the materials of the contact surfaces.
4. The friction force is independent of the area of contact surfaces.
(a) 2, 3 and 4 (b) 1 and 3
(c) 2 and 4 (d) 1, 2, 3 and 4