A child is born with cystic fibrosis, an inherited recessive disorder. No one in the family (on either side) for several generations is known to carry the gene or have the disorder. How can this be explained?

a. The genetic disorder formed from the mother's poor nutrition and prenatal care.
b. There was a mutation in the DNA, either in one of the gametes or the fertilized egg.
c. The child contracted the illness from another child when he was born.
d. The child developed the disorder when he was born; it is a result of a birth defect.