true/false

a)Force of friction icreases if the two surfaces are pressed harder.

b)Sliding or kinetic friction is smaller than the limiting value of static fraction.

c)Friction increases with height.

a) True because on pressing two surfaces the normal reaction increases and a greater force is needed to move the surfaces relative to each other.
b) True.
c) well here if you mean friction due to air then its  false because with increase in height air density decreases which results in less air drag.