阅读理解 If you're training for a marathon , a proper plan for water intake (摄入) is important . Of course , there's the risk of dehydration (脱水) . But athletes now know they can also get into trouble by drinking to much . To much water intake can lower levels of sodium (钠) in the blood . The death of a 28-year-old woman following the Boston Marathon caught the attention of many runners and led to new research . Experts advise long distance runners to replace the liquids they sweat out . Their goal is to try to keep someone from not getting dehydrated by more than 2 percent of their body weight . One technique for deciding how much water you need is to get the exact numbers . Runners have to weight themselves before and after a run to determine how much water they've lost . If their weight drops by more than 2 percent , they have not drunk enough water. Hyponatremia occurs when runners drink so much water that blood salt levels drop off . A study published last year tested 488 runners who completed the Boston Marathon and found 13 percent of them had dangerously low blood salt levels . The first sign that runners may notice is slightly swelling in the hands . They can't get their rings off , and then they might feel sick . They may not remember where they are . In fact , most runners get enough salt to get back to normal levels by eating just one meal after a run . Contrary to the old advice that runners should drink as much as they can to prevent dehydration , the new research has shown that the body is a remarkable machine that actually tells you via thirst when you need water . 1．The author of this passage is primarily concerned with        . A．the signs and treatment for dehydration B．the long distance runners who need help C．how to manage water intake during a marathon D．a 28-year-old woman who died after a Boston Marathon 2．Why do runners have to weigh themselves before and after a run ? A．They can improve their performance in a run . B．They can learn how much salt there is in the blood . C．They ca know the quantity of water they've lost during a run . D．They can decide what kind of water they should take during a run . 3．What conclusion can we draw from the passage ? A．Taking much salt during a run can avoid selling in the body . B．Either too much or to little of water intake is harmful . C．A rich meal is helpful to our performance in a run . D．We should take enough salt during a run . 4．According the passage, which of the following statements is correct ? A．Too much water intake→blood sodium levels ?→dehydration . B．Too much water intake→blood salt levels ?→hyponatremia . C．No water intake→blood sodium levels↑→hyponatremia . D．No water intake →blood salt levels ?→hyponatrenmia .