Javier and Alyssa Are Brother and Sister, and Both Have

Multiple Choice

 Javier and Alyssa are brother and sister, and both have O type blood. They know that their mother has type A blood, but do not know their father's blood type. Given this information, what could their father's blood type be?

A) could be A, B, or O
B) must be O
C) must be A
D) could be AB or O