单项选择题

Passage One Questions 11 to 13 are based on the passage you’ve just heard.

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问答题
River and lake pollution, if it is toxic enough, may kill animal and plant life immediately, or it may injure slowly.
问答题
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.
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