Meaningful studies on this topic are very difficult to perform because they require large populations of subjects and many years of data collection, and because poor health sometimes results in limitations to physical activity.

【参考答案】

就这个内容进行有意义的研究是很难的，因为它们需要大量有关的人口数据和多年的数据搜集，也因为有时不健康会导致体力活动的局限......

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