Researchers are investigating the use of gene therapy as a treatment option for human genetic disorders, such as cystic fibrosis. When gene therapy is successful, how does it treat the disorder?

A. by activating, or "turning on," one or more genes
B. by deactivating, or "turning off," one or more genes
C. by incorporating a human gene into the genomes of bacteria or other organisms
D. by replacing a faulty gene with a normal gene in the human genome