阅读下列短文，从每题所给的A、B、C、D四个选项中，选出最佳选项。
C
When John was growing up, other kids felt sorry for him. His parents always had him weeding the garden, carrying out the garbage and delivering newspapers. But when John reached adulthood, he was better off than his childhood playmates. He had more job satisfaction, a better marriage and was healthier. Most of all, he was happier. Far happier.
These are the findings of a 40-year study that followed the lives of 456 teenage boys from Boston. The study showed that those who had worked as boys enjoyed happier and more productive lives than those who had not. “Boys who worked in the home or community gained competence (能力) and came to feel they were worthwhile members of society,” said George Vaillant, the psychologist (心理学家) who made the discovery. “And because they felt good about themselves, others felt good about them.”
Vaillant’s study followed these males in great detail. Interviews were repeated at ages 25, 31 and 47. Under Vaillant, the researchers compared the men’s mental-health scores with their boyhood-activity scores with their boyhood-activity scores. Points were awarded for part-time jobs, housework, effort in school, and ability to deal with problems.
The link between what the men had done as boys and how they turned out as adults was surprisingly sharp. Those who had done the most boyhood activities were twice as likely to have warm relations with a wide variety of people, five times as likely to be well paid and 16 times less likely to have been unemployed. The researchers also found that IQ and family social and economic class made no real difference in how the boys turned out. Working----at any age----is important. Childhood activities help a child develop responsibility, independence, confidence and competence---the underpinnings (基础) of emotional health. They also help him understand that people must cooperate and work toward common goals. The most competent adults are those who know how to do this. Yet work isn’t everything. As Tolstoy once said, “One can live magnificently in this world if one knows how to work and how to love, to work for the person one loves and to love one’s work.”
What does the underlined word “sharp” probably mean in Paragraph 4?（）
A. Quick to react
B. Having a thin edge
C. Clear and definite
D. sudden and rapid