A、They tried to make the transplanting safer. B、They shifted their attention to "nonessential" parts of the body. C、They focused

Scientists have long thought about the idea of replacing a diseased organ with a healthy one from a donor. The problem at first was that the human body is not particularly receptive to foreign tissue. When tissue from a donor is placed inside the body, the immune army sees it as a foreign invader and goes into battle mode. White blood cells attack and destroy the unknown tissue in a process known as rejection.
    Eventually, scientists realized that the problem of rejection didn’t occur when the organ donor and recipient were identical twins. The genetic similarity appeared to prevent the immune response. Massachusetts surgeon Joseph E. Murray used this concept to his advantage in 1954, when he accomplished the first successful kidney transplant between identical twins.
    Dr. Murray’s surgery was a major breakthrough, but it wasn’t a solution. After all, very few people have an identical twin they can rely on for organ donation. In the late 1960s, doctors figured out a way to perform transplants between non-relatives by suppressing the recipient’s immune response with drugs. But the trouble was that the drugs were highly poisonous. Between the risks of infection and the poisonous drugs, most transplant patients didn’t live long after their operation.
    By the 1980s, anti-rejection drugs had improved to the point where transplantation surgery became pretty routine and far less risky. Survival rates rose. Once surgeons became experienced in transplanting essential organs such as heart, kidneys, liver and lungs, they turned their focus to "nonessential" parts of the body.
23. What was the problem of organ transplant at the beginning?
24. In what condition would rejection not occur?
25. What did surgeons do after they got familiar with the transplant of essential organs?