Fill in each of the 15 blanks in the passage with the most likely answer. Write the letter corresponding to your choice on the answer sheet. (1 point each, 15 points in all)
①Most living things undergo two types of biological change during their lifetime. One is （1） , an increase in the size or efficiency of an organism. The other is aging, which involves a decrease in size or efficiency.
Signs of aging begin to （2）in most people between the age of 30 and40. Heredity determines most of the ways a person changes while aging, but environment also（3）. Graying hair is probably the most common sign of aging. All the senses decline with age. For example, the eyes lose their ability to adapt （4） darkness, and they require brighter light for reading.
②Movement becomes harder with age. By age 80, about half the muscle cells have been replaced by other kinds of tissue. In women（5）, bones lose calcium and become more likely to （6） . As a person ages, the body's ability to combat infection declines. This change occurs because the white blood cells of （7） is called the body's immune system lose their （8） function.
③Many people believe that with advancing age, an individual loses the ability to learn, remember, and make decisions. But（9） disease or injury damages the brain, a healthy elderly person who（10） active probably suffers no serious decline in （11）ability.
④No one completely understands the aging process. Some scientists（12） that human aging will ever be controlled. Others believe that aging can be（13） . Usually, normal human cells can be kept（14） in a laboratory for only a limited time. But（15）cells, such as various kinds of cancer cells, can live for a long time. If scientists can determine how such cells survive, they may gain an insight into the process of cell aging.
第3题
A、plays a role
B、leads a way
C、makes a decision
D、offers a chance
【正确答案】：A
【题目解析】：遗传决定了一个人在衰老过程中的变化，环境又起着重要作用。根据上下文可知选A。