Determine which lines, if any, are parallel. Choices may be repeated and some might not be used at all. Question 6 options: m∠20 + m∠21 = 180° ∠13 ≅ ∠17 ∠4 ≅ ∠9 m∠14 + m∠17 = 180° ∠10 ≅ ∠23 ∠8 ≅ ∠19 1. a || b 2. b || c 3. a || c 4. d || e 5. doesn't prove lines parallel