单选题
Recent studies of the human brain have resulted in some interesting discoveries. Scientists believe that a way to improve the power of the brain may soon be possible. Scientists have discovered that the brain can make its own drugs. The brain【C1】______a protein substance which can act directly【C2】______the brain to change aspects of mental activity. Some may change or improve, for example, creativity, intelligence, imagination, and good【C3】______. Chemicals found in the brain【C4】______messages. In recent years scientists have found chemicals that【C5】______mood, memory and other happenings of the mind. About 25 have been found【C6】______. Today the role of chemicals and the protein substance in human behavior is creating much interest. Research seems to show that they may help【C7】______insomnia, pain, and mental illness. They have a great【C8】______to stimulate the brain to【C9】______deficiencies. They also improve the qualities of memory and learning already in the brain. They【C10】______the secret on mood and emotion. Some day there may be a chemical way to create a better and more efficient brain.