Researchers are investigating the use pf 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" on or more genes B. by deactivating or "turning off" on 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