The number of people diagnosed as having a certain intestinal disease

Dear longhaul123,

I see that abhimahna answered your question, but I am going to add a few thoughts as well.

Here's (B)
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.
First of all, the wording is very tricky. What it is saying is that:
People with the intestinal disease didn't drink a lot of bottled water.
People without the intestinal disease drank a lot of bottled water.

Also, notice how you changed the logic. You spoke of "Bottled spring water that is from the water treatment plant." My friend, with all due respect, this is a major misunderstanding. This is an example of the real-world experience you need to answer GMAT CR questions. Water that comes from a water-treatment plant is NOT spring water. Also, by its very nature, a water-treatment plant is not a bottling facility: nothing is bottled there. A spring is a place where water flows up naturally from the ground: many times, this is some of the purest water one could possibly drink. Many companies (Arrowhead, Volvic, Iceland, Alhambra, Poland Springs, Crystal Geyser, etc.) bottle spring water in plastic bottles, and these plastic bottles of water are available to consumers. This is a very different source of water than the water that comes out of the tap. Water from a water-treatment facility would be water out of the tap in people's house. Some people don't trust tap water, so they go to the store to buy bottled spring water. Other people point out that tap water is held to strict standards and bottled water isn't; also, the plastic bottles aren't healthy because they leech chemicals into the water. This has been a major debate in the USA for several years.

In order to do well on the GMAT CR, you need to read the news and be aware of issues in the real world. See:
GMAT Critical Reasoning and Outside Knowledge

Thus, what we learn is that the people who were drinking tap water, the water that had come from the water-treatment facility, got the intestinal disease, but the folks who avoided tap water and instead bought & drank bottled spring water did not get the disease. Thus, the source of the disease originated in the water-treatment plant. This would be a strengthener when you understand what it is saying. It's very typical for a strengthener to be a trap answer for a weakener question.

Here's (C)
(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.

This "new diagnostic technique" corrected diagnoses.

Last year, a large number of people were diagnosed with this intestinal disease. That's part of the evidence. What is not discussed, though, in the prompt is whether they were diagnosed correctly. That was before the "new diagnostic technique." (C) tells us that before this "new diagnostic technique," people were misdiagnosed with the intestinal disease. Thus, it could be that the high number of diagnoses of the intestinal disease was due, in part, to a large number of misdiagnoses, people who actually had intestinal ulcers, but the medical technology available at the time incorrectly ascribed their symptoms to the intestinal disease.

This also would explain why, when the "new diagnostic technique" was introduced this year, why the number of diagnoses of the intestinal disease was lower this year--with the new procedure, there was no confusion. The people suffering from intestinal ulcers were no longer misdiagnosed as having the intestinal disease.

This is a truly brilliant OA. It's not that the actual number of cases of the intestinal disease decreased at all. Instead, the drop was due to a correction of a measurement error: other diseased were being misdiagnosed as the intestinal disease, inflating the number of those cases, so when all the diagnoses were corrected, the low number of cases of the intestinal disease was more in line with reality.

Does all this make sense?
Mike :-)