PQ and RS are two lines that intersect at point T, as shown below: Two lines PQ and RS intersect at point T. Angles PTR and STQ are shown congruent.

Which statement is used to prove that angle PTR is always equal to angle STQ?

choices:

a. Lines PQ and RS do not have a fixed length.

b. Angle PTR and angle PTS are supplementary angles.

c. Lines PQ and RS intersect at an angle less than a right angle.

d. Angle PTR and angle PTS are complementary angles.