Consider the following statements about the ABO blood grouping in human beings and select the incorrect pair from the given options 
a. ABO blood groups are controlled by gene I, having three alleles I^A, I^B and i.
b. I^A and I^B are completely dominant over i but I^A and I^B show incomplete dominance when they are present together
c. The alleles I^A and I^B produce almost similar form of sugar but the i allele produce a completely different sugar
d. A total of six different genotypes and four phenotypes are possible