Two black Guinea pigs are mated together on several occasions. Their offsprings are invariably black. However, when their black offsprings are, mated wit white Guinea pigs, half the matings results in litters containing black babies only and the other half equal numbers of black and white babies. If colour is determined by a single pair of alleles, the genotypes of the two parents are
A. Homozygous dominant and heteroxygous
B. Homozygous dominant and homzygous recessive
C. both homozygous dominant
D. both heterozygous
