Theorem: the segment joining the midpoints of two sides of a triangle is parallel to the third side and half its length. a two-column proof of the theorem is shown, but the proof is incomplete. a triangle with vertices a is at 6, 8. b is at 2, 2. c is at 8, 4. segment de with point d on side ab and point e is on side bc statement reason the coordinates of point d are (4, 5) and coordinates of point e are (5, 3) length of segment de is square root of 5 and length of segment ac is 2 multiplied by the square root of 5 by the distance formula segment de is half the length of segment ac by substitution slope of segment de is −2 and slope of segment ac is −2 by the slope formula segment de is parallel to segment ac slopes of parallel lines are equal which of the following completes the proof? (6 points) select one: a. by the midpoint formula b. by definition of congruence c. given d. by construction