In a maternity ward, four babies are accidentally mixed up. The ABO blood types of the four babies are known to be O, A, B, and AB. The ABO blood types of the fur sets of possible parents are determined, and used to determine which baby belonged to which parents. Which baby belongs to each of the following parents?

(a) AB x O
(b) A x O
(c) A x AB
(d) O x O