Fill in the blank with the phrase that makes the proof statement true. lines m and n are parallel. angle 1 is congruent to angle 2 by the corresponding angles postulate. angle 1 is congruent to angle 3 because they are vertical angles. by the transitive property. a. angle 2 is congruent to angle 3 b. angle 3 is supplementary to angle 1 c. angle 2 is congruent to angle 1 d. angle 1 is complementary to angle 2