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.
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.