Under What Circumstances Might Identical Twins Be Conjoined

Under what circumstances might identical twins be conjoined?

A) If there is some mutation in the genetics of the mother.
B) If there is some mutation in the genetics of the father.
C) If two sperm fertilize the same egg.
D) If the egg splits in two very early after fertilization (within five days) .
E) If the egg splits relatively late after fertilization (after nine days) .