Women (and all female mammals)have one active X chromosome per cell instead of two.What causes this?

A)modification of the XIST gene so that it is active only on one X chromosome, which then becomes inactive
B)activation of the Barr gene on one of the two X chromosomes that then inactivates
C)crossover 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)the removal of methyl (CH₃)groups from the X chromosome that will remain active