In human ABO blood group system,there are six possible genotypes but only four phenotypes.An explanation of this is that the ABO blood groups are controlled by

Options

A. one gene locus with three incompletely dominant (co-dominant)alleles.
B. one gene locus with two incompletely dominant (co-dominant)alleles and two recessive alleles.
C. one gene locus with two incompletely dominant (co-dominant)alleles and one recessive allele.
D. two unlinked gene loci each with two alleles,one dominant and one recessive.