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13.  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.（C）

    

(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：C

题目说，诊断出得肠病的人比去年少了，原因是水处理厂的卫生改善了，水更干净了所有病就少了。
答案C说的是什么啊？不懂

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为什么没有人理我？？？