A、By taking the blood out of the brain. B、By lowering the brain’s temperature. C、By trying the operation on monkeys first. D、By

It is difficult for doctors to help a person with a damaged brain. Without enough blood, the brain lives for only three to five minutes. More often the doctors can’t fix the damage. Sometimes they are afraid to try to do something to help because it is dangerous to work on the brain. The doctors might make the person worse if he operates on the brain.
    Dr. Robert White, a famous professor and doctor, thinks he knows a way to help. He thinks doctors should make the brain very cold. If it is very cold, the brain can live without blood for 30 minutes. This gives the doctor a longer time to do something for the brain.
    Dr. White tried his idea on 13 monkeys. First he taught them to do different jobs. Then he operated on them. He made the monkeys’ blood go through a machine. The machine cooled the blood. Then the machine sent the blood back to the monkeys’ brains. When the brain’s temperature was 10℃, Dr. White stopped the blood to the brain. After 30 minutes he turned the blood back on. He warmed the blood again. After the operations the monkeys were all like they had been before. They were healthy and busy. Each one could still do the jobs the doctor had taught them.
Questions 20 to 22 are based on the passage you have just heard.
20. What is the biggest difficulty in operating on the damaged brain?
21. By mainly what was the brain operation made possible?
22. What do we learn about the operation on the damaged brain according to Dr. White’s new idea?