A、Numbers of people involved. B、Quality of this relationship. C、Commitment of this relationship. D、Members being relatives. B根据第

The Harvard Study of Adult Development may be the longest study of adult life that’s ever been done. [16] For 75 years, we’ve tracked the lives of 724 men, year after year, to find out one thing: What keeps us healthy and happy as we go through life?
    To get the clearest picture of these lives, we don’t just send them questionnaires. We interview them in their living rooms. We get their medical records from their doctors. We draw their blood, we scan their brains, and we talk to their children. We videotape them talking with their families about their deepest concerns.
    So what have we learned? Well, the lessons aren’t about wealth or fame or working harder and harder. The clearest message that we get from this 75-year study is this: Good relationships keep us happier and healthier.
    We’ve learned three big lessons about relationships. The first is that social connections are really good for us. People who are more socially connected to family, to friends, to community, are happier, they’re physically healthier, and they live longer than people who are less well connected. And the experience of loneliness turns out to be bad. People who are more isolated than they want to be from others find that they are less happy, [17] their health declines earlier in midlife.
    And we know that you can be lonely in a crowd and you can be lonely in a marriage, so the second big lesson that we learned is that it’s not just the number of friends you have, and it’s not whether or not you’re in a committed relationship, [18] but it’s the quality of your close relationships that matters. High-conflict marriages, for example, without much affection, turn out to be very bad for our health, perhaps worse than getting divorced. And good, close relationships seem to shelter us from some of the hardship of getting old. Our most happily partnered men and women reported, in their 80s, that on the days when they had more physical pain, their mood stayed just as happy.
    And the third big lesson that we learned about relationships and our health is that good relationships don’t just protect our bodies, they protect our brains. And those good relationships, they don’t have to be smooth all the time. Some of the couples in their 80s or 90s could quarrel with each other day in and day out, [19] but as long as they felt that they could really count on the other when the going got tough, those quarrels didn’t do huge harm on their memories.
    So this message, that good, close relationships are good for our health and well-being, this is wisdom that’s as old as the hills.
    16. What did the Harvard Study of Adult Development try to find out?
    17. What is the study’s conclusion about loneliness?
    18. What is the most important for a relationship according to the second lesson?
    19. Why don’t small quarrels among old couples affect their relationship?