All Female Mammals Have One Active X Chromosome Per Cell

Multiple Choice

All female mammals have one active X chromosome per cell instead of two. What causes this?

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
E) attachment of methyl (CH3) groups to the X chromosome that will remain active