What Event Causes All Female Mammals to Have One Active

Multiple Choice

What event causes all female mammals to have one active X chromosome per cell instead of two?

A) activation of the XIST gene on the X chromosome that will become the Barr body
B) activation of the BARR gene on one X chromosome, which then becomes inactive
C) crossing over between the XIST gene on one X chromosome and a related gene on an autosome
D) inactivation of the XIST gene on the X chromosome derived from the male parent