Questions 18—21 are based on the following dialogue between a reporter and a health expert．
Who are the research subjects of Dr．Stone’s study?

Questions 18-21
W: Dr. Stone, you have studied 25 people who never get sick. Could you give us some easy-to-follow tips on how they' ve successfully stayed away from illness in their whole lives?
M: Our study shows that some habits will help you to stay well and live longer. Such as, cold shower, napping, weightlifting and swim-ming.
W: Do you mean that taking a cold shower is good to our health?
M: Yes. Professor Smith is 82 years old now. He has taken a cold show-er every day for most of his adult life, and never suffers from colds.
W: That sounds great. Maybe I can try it myself. What else can we do to keep us against illness?
M: Besides exercise, friends are also very important. You might not think having a social network would count as much as diet and exer-cise when it comes to good health. But studies have found that good friends can protect against everything from memory-lost to heart at-tacks, to infectious diseases. Mary is a former nurse. She can' t re-member being ill in all her 75 years, and she credits this to the fact that she is constantly seeking out new friends. Sociability increases self-respect which in turn supports the immune system.