Which of the following statements are true regarding C4? (Select all that apply.)

Which of the following statements are true regarding C4? (Select all that apply.)
 
  a. There are two forms of C4 encoded by separate genes residing in the class II region of the MHC.
  b. Evolution of the different forms of C4 probably occurred as a result of gene duplication and diversification.
  c. Because there are two forms of C4, C4 deficiency is the least common human immunodeficiency.
  d. More than 30 of the human population lacks either C4A or C4B.
  e. C4A and C4B have identical properties.
  f. C4B deficiency is associated with increased susceptibility to systemic lupus erythematosus.
  g. C4A and C4B are monomorphic.