Human Blood Type Is Determined by Three Alleles I^A, I^B

Human blood type is determined by three alleles I^A, I^B, and I^O. The alleles I^A and I^B are co-dominant to each other, and both are dominant to I^O. Within a large, randomly mating population (540,000 individuals) , the frequencies for the blood type alleles are 0.3 for the I^A allele, 0.6 for the I^O allele, and 0.1 for the I^B allele. What percentage of the type B people are heterozygotes (I^BI^O) ?

A) 12%
B) 13%
C) 92.3%
D) 6%
E) 60%