Two blocks P (upper) and Q (lower) are kept one above the other on a smooth horizontal table. A horizontal force F pulls the lower block so that both blocks move together without relative motion. The friction acting at the interface between P and Q is
a) 0 (no friction) because the surfaces are not sliding relative to each other
b) a pair of static friction forces acting on the two blocks in opposite directions
c) a static friction force acting only on the upper block toward the pull
d) a pair of kinetic friction forces acting on the two blocks opposite to their motion