Arelation  r  is defined by this list of ordered pairs:         (4, 6), (5, 7), (3, 7), (4, 6), (5, 5), (6, 4) what is the domain of relation  r? a.{4, 5, 3, 4, 5, 6} b.{6, 7, 7, 6, 5, 4} c.{3, 4, 5, 6} d.{4, 5, 6, 7}