Which relation on the set {1, 2, 3, 4} is a partial ordering?
a. { (1, 2), (2, 3), (1, 3), (4, 3) }
b. { (1, 2), (2, 3), (1, 3), (3, 4) }
c. { (1, 2), (2, 3), (1, 3), (3, 4), (1, 1), (2, 2), (3, 3), (4, 4) }
d. { (1, 2), (2, 3), (1, 3), (4, 3), (1, 1), (2, 2), (3, 3), (4, 4) }