A、Current smokers exclusively. B、Second-hand smokers. C、With a lung problem. D、At age 40 or over. D题目问的是研究的参与者其中一个要求是什么。由定位句可知，研

For years researchers have debated whether smoking effects the lungs in men and women differently. In a most compelling study on the topic to date, researchers determined that women are twice as vulnerable to lung cancer as men. But in a surprising twist, they die at half the rate of men. The study, which was published last week in the Journal of the American Medical Association (J. A. M. A.), included 9427 men and 7498 women from throughout North America who were healthy, at least 40 years old and either current or former smokers. Over the course of more than eight years, a group of investigators led by Dr. Claudia Hensch-ke of the Weill Medical College in New York City identified lung tumors in 113 of the men and 156 of the women. Then the researchers kept track of who lived and for how long, as well as the treatment participants were given. The study showed that both sexes tended to be in their late 60s when they received a lung-cancer diagnosis but that the women usually had smoked considerably less than the men.
    Q28: What was one of the requirements for the participants of the study?
    Q29: Over the course of more than eight years, how many of the participants developed lung cancer?
    Q30: Which of the following is one finding of the study?