听力原文： Hi, there. Let me make a brief introduction of the brain first. The brain tells the other parts of the body what to do. We think with our brain. We understand the world around us with our brain. However, doctors don't know much about how the brain works. Doctors can do many things with other parts of the body. They can give a person a new heart. They can put an arm back on 'after it has been cut off. It is easy to study some parts of the body. However, it is difficult to know how the brain works. It is most difficult to work on a brain. If the brain is without enough blood for three to five minutes, the brain dies. So when doctors work on the brain, they only have three to five minutes to do their work] Doctors try new things on the brain, but they must be very careful. The brain is the center of those things that make us what we are. Which of the following can't doctors do?