Two Parents with Type a and Type B Blood Have

Two parents with Type A and Type B blood have a child with Type AB blood. How is this possible?

A) Type A blood is codominant with Type B blood, so both alleles are expressed equally.
B) Type A blood is incompletely dominant to Type B blood. This means that individuals with alleles for both Type A and Type B will express a blended intermediate between these two blood types.
C) This is not possible. The child cannot be their biological child.
D) Type A blood is co-recessive with Type B blood, so as long as there is not a dominant blood type present, both recessive alleles will be expressed equally.