Thomas needs to prove the following theorem: If one pair of opposite sides of a quadrilateral is congruent and parallel, then the quadrilateral is a parallelogram.
He draws the figure below and begins his proof.

Prove: ABCD is a parallelogram

What should be Thomas's reason for Step 2?

If parallel lines are cut by a transversal, corresponding angles are congruent.

A) Vertical angles are congruent.
B) If parallel lines are cut by a transversal, alternate interior angles are congruent.
C) Congruent parts of congruent triangles are congruent
D) If parallel line are cut by a transversal, corresponding angles are congruent