fall asleep.
主题句的位置可以在段尾：
doctors are of the opinion that most people cannot live beyond 100 years，but a growing number of scientists believe that the aging process can be controlled.there are more than 12 000 americans over 100 years old，and their numbers are increasing each year.drjames langley of chicago claims that，theoretically and under ideal（理想的）conditions，animals，including man，can live six times longer than their normal period of growth.a person‘s period of growth lasts about 25 years.if dr.langley’s theory is accurate（准确的）， future generations can expect a life span（寿命）of 150 years 。 |
