Why Is Dosage Inconsistent in the Duffy Blood Group

Why is dosage inconsistent in the Duffy blood group?

A) The genotype could be inherited as Fy^aFy^b or Fy^aFy for Fy(a+b-) red blood cells.
B) Antigen expression is depressed in low ionic strength media.
C) The genotype could be inherited as Fy^aFy^a or Fy^aFy for Fy(a+b-) red blood cells.
D) All of the above