Pleas help ASAP!! Janet is designing a frame for a client. She wants to prove to her client that m∠1 = m∠3 in her sketch. What is the missing justification in the proof?

Segment CD intersects segment AB, creating angles 1, 2, 3, and 4.

Statement Justification
Segment CD intersects segment AB Given
∠1 + ∠ 2 = 180°
∠ 2 + ∠3 = 180°
∠1 + ∠ 2 = ∠ 2 + ∠3 Transitive Property
∠1 = ∠3 Subtraction Property
Definition of Supplementary Angles
Intersecting Lines Postulate
Same-Side Interior Angles Theorem
Vertical Angles Theorem