IKMC 2007 - Grades 9-10 - Self-study Question 8

Points A (2006,2007), B (2007,2006), C (−2006, −2007), D (2006 – 2007), E (2007, −2006) are marked. Which segment is horizontal? Source note: D lacks a coordinate separator; the intended pair is D (2006, −2007).