Why Is It Possible for Two Healthy Parents to Give

Why is it possible for two healthy parents to give birth to a child with a genetic defect such as cystic fibrosis?

A) CF is normally caused by a mutation that occurs only in the sperm and eggs of the parents,so their bodies are not affected.
B) CF is normally caused by a mutation that occurs in the fetus during pregnancy,and it does not affect the mother.
C) CF is only evident in individuals with two mutant alleles;a healthy parent could carry both a defective allele and a normal allele.
D) CF is not passed to children from their parents;it comes from their grandparents.
E) CF is caused by the inheritance of too many "normal" genes;the parents would be unaffected.