a. When P gets closer to Q, the distance of the line gets smaller. As P gets farther from Q, the distance of the line gets larger. The smallest distance possible is 0. When the distance between the two points is zero, the points P and Q must be the same and have the same coordinate.
b. It does not matter which point you start with, so PQ is the same distance as QP.
c. PR is less that PQ+QR. Our observation holds for the cases that we have seen.