Which statement must be true to prove m∠1 + m∠2 + m∠3 = 180°? Triangle ABC is drawn such that point C lies on line m and side AB lies on line n. Line m and line n are parallel to each other. Measure angle A is marked as 1, angle C is marked as 3 and exterior angle to angle C is marked as 2. A. m ∥ n B. m∠1 + m∠2 = 180° − m∠3 C. m∠1 + m∠2 = 90° D. m∠1 = m∠2 = m∠3