A、724. B、90. C、60. D、2,000.00 C演讲中提到，“在最早的724名男性中，大约有60位还在世，并继续参与这项研究，他们绝大多数都已经超过90岁了”。故选C。

What keeps us healthy and happy as we go through life? If you were going to invest now in your future best self, where would you put your time and your energy? [20] There was a recent survey of millennials asking them what their most important life goals were, and over 80 percent said that a major life goal for them was to get rich. And another 50 percent of those same young adults said that another major life goal was to become famous.
    And we’re constantly told to lean in to work, to push harder and achieve more. We’re given the impression that these are the things that we need to go after in order to have a good life. Pictures of entire lives, of the choices that people make and how those choices work out for them, those pictures are almost impossible to get. Most of what we know about human life we know from asking people to remember the past, and as we know, hindsight is anything but 20/20. We forget vast amounts of what happens to us in life, and sometimes memory is downright creative.
    But what if we could watch entire lives as they unfold through time? What if we could study people from the time that they were teenagers all the way into old age to see what really keeps people happy and healthy?
    We did that. The Harvard Study of Adult Development may be the longest study of adult life that’s ever been done. [21] For 75 years, we’ve tracked the lives of 724 men, year after year, asking about their work, their home lives, their health, and of course asking all along the way without knowing how their life stories were going to turn out.
    Studies like this are exceedingly rare. Almost all projects of this kind fall apart within a decade because too many people drop out of the study, or funding for the research dries up, or the researchers get distracted, or they die, and nobody moves the ball further down the field. But through a combination of luck and the persistence of several generations of researchers, this study has survived. [22] About 60 of our original 724 men are still alive, still participating in the study, most of them in their 90s. And we are now beginning to study the more than 2,000 children of these men. And I’m the fourth director of the study.
    Since 1938, we’ve tracked the lives of two groups of men. The first group started in the study when they were sophomores at Harvard College. They all finished college during World War II, and then most went off to serve in the war. And the second group that we’ve followed was a group of boys from Boston’s poorest neighborhoods, boys who were chosen for the study specifically because they were from some of the most troubled and disadvantaged families in the Boston of the 1930s. Most lived in tenements, many without hot and cold running water.
    When they entered the study, all of these teenagers were interviewed. They were given medical exams. We went to their homes and we interviewed their parents. And then these teenagers grew up into adults who entered all walks of life. They became factory workers and lawyers and bricklayers and doctors, one President of the United States. Some developed alcoholism. A few developed schizophrenia. Some climbed the social ladder from the bottom all the way to the very top, and some made that journey in the opposite direction.
20. What is the most important life goal of the majority of millennials?
21. Which topic is not being asked in the study?
22. How many original respondents are still alive?