How Are Twins Indicated on a Pedigree

How are twins indicated on a pedigree?

A) The two gender symbols are placed on top of each other.
B) Each of the gender symbols is surrounded by an inward-facing set of brackets.
C) The vertical lines to each twin begin at the same point on the parents' horizontal mating line.
D) The one born first has a vertical line connected to the parents' horizontal mating line; the twin born second has a vertical line extending down from the first twin's gender symbol.