17. No, it isn't.
18.The old "8-a-day" advice meant, is it necessary to drink down eight glasses of water daily, or is this already out of date.?
19.The old "8-a-day" rule is for people who is under difficult situations.For example,soldiers at high altitude and hospitalized patients.
20. 依据作者的看法，您本身真正需要多少分量的水呢？（自行回答）
21. The article is mainly about how much water do we really need daily.
我不确定答案是否正确，希望对您有帮助。