All female mammals have one active X chromosome per cell instead of two. What causes this to happen? (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) inactivation of the XIST gene on the X chromosome derived from the male parent (D) attachment of methyl (CH₃) groups to the X chromosome that will remain active