Given: ΔABC Prove: The sum of the interior angle measures of ΔABC is 180°. Statement Reason 1. Let points A, B, and C form a triangle. given 2. Let be a line passing through B, parallel to , with angles as labeled. defining a parallel line and labeling angles 3. 4. m∠1 = m∠4, and m∠3 = m∠5. Congruent angles have equal measures. 5. m∠4 + m∠2 + m∠5 = 180° angle addition and definition of a straight angle 6. m∠1 + m∠2 + m∠3 = 180° substitution What is the missing step in this proof? A. Statement: ∠4 ≅ ∠5, and ∠1 ≅ ∠3. Reason: Alternate Interior Angles Theorem B. Statement: is parallel to . Reason: is a transversal cutting and . C. Statement: ∠1 ≅ ∠4, and ∠3 ≅ ∠5. Reason: Alternate Interior Angles Theorem D. Statement: ∠1 ≅ ∠4, and ∠3 ≅ ∠5. Reason: ∠1 and ∠4, and ∠3 and ∠5 are pairs of supplementary angles.