Critical Reasoning (CR) | Re: The number of people diagnosed as having a certain intestinal disease

Even after going through all the explanations i am still not understanding why B must not the answer. Because option B says that the Bottled spring water that is from the water treatment plant is doing nothing to the people diagnosed with the disease and for people who have not contracted such disease. So this statement says that the water treatment plant does nothing to reduce the intestinal disorder and hence weakens the official's conclusion.
But where as the option C talks about the new diagnostic