Imagine two planes: they can be parallel or not. If they are parallel, then they don't have common points, but your planes have common points P and Q.
So, you can conclude that these two planes are not parallel, then they should intersect. Two planes intersect <span>in a straight line and this line consists of all common points of both planes. </span> If planes are named BPQ (points B, P, Q do not lie on one straight line) and CPQ (points C, P, Q do not lie on one straight line), you can see two common points P and Q. Hence the intersection line must contain these two points and may be named as PQ (and points B and C do not lie on this line). Answer: correct choice is C.
