Fill in the missing reason in the proof
A) Definition of straight angles
B) Alternate Interior Angles are congruent
C) Supplementary angles sum up to 180°
D) Complementary Angles sum up to 180°





ABC is a triangle Given
DE ||AC Given
∠ A = ∠ABD Alternate interior angles are congruent
∠ C = ∠CBE Alternate interior angles are congruent
∠ABD + ∠B + ∠CBE = 180° ?
∠A + ∠B + ∠C = 180° Substitution