Cystic fibrosis is a disorder that causes the mucus glands to produce very thick mucus. Problems with the respiratory and digestive systems often occur in individuals with this condition. Like many disorders, cystic fibrosis occurs when there is a simple change, or mutation, in the DNA of a particular gene. According to the information above, genetic mutations

A. usually result in the release of excess mucus.

B. can only cause diseases in plants and wild animals.

C. will always cause death in humans.

D. can contribute to or cause diseases in humans.