29. The number of people diagnosed as having a certain intestinal disease has dropped significantly in a rural county this year, as compared to last year. Health officials attribute this decrease entirely to improved sanitary conditions at water-treatment plants, which made for cleaner water this year and thus reduced the incidence of the disease.
Which of the following, if true, would most seriously weaken the health officials’ explanation for the lower incidence of the disease?
A. many new water-treatment plants have been built in the last five years in the rural county.
B. Bottled spring water has not been consumed in significantly different quantities by people diagnosed as having the intestinal disease, as compared to people who did not contract the disease.
C. Because of a new diagnostic technique, many people who until this year would have been diagnosed as having the intestinal disease are now correctly diagnosed as suffering from intestinal ulcers.
D. Because of medical advances this year, far fewer people who contract the intestinal disease will develop severe cases of the disease.
E. The water in the rural county was brought up to the sanitary standards of the water in neighboring counties ten years ago.

答案为Ｃ．

关于Ｂ，bottled spring water是不是属于干净的水？我觉得是，那么Ｂ的意思，people diagnosed as having the intestinal disease与people who did not contract the disease所consume的这种水的量没有多少区别．那么就是说，水的干净与否并没有改变人们的发病率，那么不就是weaken了题干的说法吗？题干说，improved sanitary conditions at water-treatment plants made cleaner water this year and thus reduced the incidence of the disease．

大家帮忙看看，真的搞不懂．

C是非常明显的他因削弱，新的诊断技术提高了诊断准确率，原来诊断为I病的人其实得了intestinal ulcers，所以I病人数减少了。

B有问题，得病和没得病的人消费这种水量相同，病肯定不是因为水造成的。（个人意见）