36. What is the main idea of the passage? (A) After two decades of lab research, gene therapy in humans has been successful in treating everything from cancer to Alzheimer's to heart disease. (B) A lot of research in gene therapy has focused on getting a gene to its destination and coaxing it to turn on once inside a cell. (C) Although research has been done on how gene therapy can help treat serious diseases, such a treatment may cause irreversible side effects and unknown consequences. (D) Viral vectors like Trojan horses slip genes into target cells undetected.