Individuals Heterozygous for the RB⁺ and RB^- Alleles Can Develop

Individuals heterozygous for the RB⁺ and RB^- alleles can develop tumors as a result of

A) a mitotic crossover that leads to homozygosity for both RB⁺ and RB^-.
B) a somatic mutation in the RB⁺ allele that leads to homozygosity for RB^-.
C) a somatic mutation in the RB^- allele that leads to homozygosity for RB⁺.
D) the fact that RB^- is dominant to RB⁺.
E) a mitotic crossover that leads to homozygosity for both RB⁺ and RB^- and a somatic mutation in the RB⁺ allele that leads to homozygosity for RB^-.