What is the equation for the line of reflection?

On a coordinate plane, image A B C D has points (2, 2), (7, 3), (5, 1), (4, 1).
Image A prime B prime C prime D prime has points (10, 2), (5, 3), (7, 1), (8, 1).

A.) x = 6
B.) y = 6
C.) y = x
D.) y = 2