You Were Recently Introduced to Your Roommate's Sibling

Multiple Choice

 You were recently introduced to your roommate's sibling. To your surprise, even though the siblings are the same sex, they look nothing alike. Which of the following is the best explanation of this situation?

A) They must have different parents, as genetic inheritance ensures that they share a significant number of genes.
B) Although they started out very similar genetically, their genotypes have changed during development to make them look very different.
C) Given the number of different combinations of genes possible during reproduction, it may be that the two siblings don't really share many alleles.
D) One has expressed only recessive genes, while the other has not.