I. The frictional force is independent of the area of the surfaces in contact
II. The frictional force depends on the nature of the surfaces in contact.
III. The frictional force depends on the speed of the sliding.
IV. The frictional force is directly proportional to the normal reaction.
Which combination of the above is true of sliding friction?

A. I, II and IV  
B. I, II and III
C. I, III and IV
D. II, III and IV

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Correct Answer: A