A、Exercise neither too little nor too much. B、Exercise without restrictions in diet. C、Exercise around a half-hour a day. D、Exer

(16)As with so many other things in life, exercise may work best if you follow the rule: exercise neither too little nor too much, if your goal is to shed extra weight, a new study finds.
     (17)Previous research has shown that exercise alone doesn’t reliably lead to weight loss—without accompanying restrictions in diet— a dismaying fact that many hopeful weight-losers know firsthand. But a recent study suggests that physical activity can indeed help shrink your pants size, so long as you hit the sweet spot— perhaps somewhere around a half hour a day, at least for young men.
    Lead researcher Rosenkilde had some theories.(18)First, the intense exercisers were probably compensating for their extra activity by eating more food. Although the added consumption noted in their food diaries wasn’t enough to explain their smaller-than-expected weight loss, Rosenkilde thinks they were likely eating more food than they wrote briefly.(19)In addition, data from the motion sensors showed that the men who exercised the most spent most of their free time sitting, probably because they were tired.
    In contrast, the men who exercised for 30 minutes at time became more active throughout the day, probably opting to take the stairs instead of the elevator, for instance, and moving more in general, Rosenkilde said. " It was little things, but they add up," Rosenkilde said.
    Overall, Rosenkilde concluded, people who exercise less may end up burning just enough calories to lose weight, but not enough to feel compelled to replace them, either by eating more or remaining sitting otherwise. Those who exercise a lot, on the other hand, may feel more drained, which prompts them to compensate.
16. What is this passage mainly talking about?
17. According to the previous research, what should people do to lose extra weight?
18. What would the intense exercisers do to make up for their extra activity?
19. How did the men who exercised the most spend most of their free time?