Identify the correct 4-tuples in the relation (a, b, c, d) ? a, b, c, and d are positive integers with abcd = 6. (check all that apply.)
a) (1, 1, 6, 1)
b) (1, 2, 3, 1)
c) (2, 1, 1, 3)
d) (2, 3, 1, 1)