A nurse is counseling a couple whose child has been diagnosed with cystic fibrosis. They understand ...

A nurse is counseling a couple whose child has been diagnosed with cystic fibrosis. They understand that this is an inherited disease, but don't know how the child got it, as neither of them is affected. What response by the nurse is best?
 
  A.
  Are you certain that you (points to man) are the biological father?
  B.
  Maybe each of you has a mild case that hasn't been diagnosed yet.
  C.
  Something in your environment must have altered one of the genes.
  D.
  This is a recessive disorder, meaning that each of you is just a carrier.