Three distinct lines a, b, and c are intersected by a single transversal t at points P, Q, and R respectively, in that order along t. At each intersection four angles are numbered as follows: 1 (upper left), 2 (upper right), 3 (lower right), and 4 (lower left). It is given that ∠4 at P is equal in measure to ∠2 at R. Which one of the following conclusions is necessarily true?
a) a∥b
b) b∥c
c) a∥c
d) None of these