When twins are genetically identical,they

A) come from the same fertilized egg.
B) come from two different fertilized eggs.
C) are not the same sex.
D) None of these.

Twins that are genetically identical, also called monozygotic twins, come from the same fertilized egg that splits into two separate embryos. This is why they have the same genetic makeup. Option B (come from two different fertilized eggs) describes dizygotic twins, which are fraternal twins that come from two separate eggs fertilized by two different sperm. Option C (are not the same sex) is not necessarily true for either monozygotic or dizygotic twins, as they can be the same or different sexes. Therefore, option D (None of these) is incorrect.