Although water in deep aquifers does not contain disease-causing bacteria, when public water supplies are drawn from deep aquifers, chlorine is often added to the water as a disinfectant because contamination can occur as a result of flaws in pipes or storage tanks. ██ ██ ██████████████ ████ ███ ██████ █████ ████ ███ ████ ████ ████████ ██ ███████████ █████ █████ ███ ██ ███ ████ ███ █████ ██ ███ ██ ███ ██████████████ ███ ███ ████████ ██████████████ █████████ ███ ████████████ ███ ███ █████ ████████ ██ ███ ██ ██████████████ ████ ███ ███ ██████████ ███ ████ █████████ █████████████ ████ ███ █████ ████████ ██ ███ ██████████████ ████ █████ █████████
The stimulus talks about municipal water supplies and disinfection. We start out with some general information:
(1) water from deep aquifers doesn't contain harmful bacteria;
(2) however, water can become contaminated by flawed pipes or storage tanks;
(3) for that reason, cities using water from deep aquifers might still add chlorine as a disinfectant.
Then, the stimulus gives us some facts about a group of 50 cities that all drew water from the same deep aquifer. All of these cities met the regional cleanliness standard, but there were also some differences:
(4) 30 cities chlorinated their water, whereas 20 cities did not;
(5) the 30 chlorinating cities had more bacterial contamination than the 20 cities that didn't chlorinate.
Which one of the following, ██ █████ ████ █████ ███████ ███ ██████████ ██ █████████ █████████████ ██ ███ ███ ██████ ██ ███████████████
Chlorine is considered ██ ████ ███████ ██ ██ █████████ ██ █████ ███████ ████ ██ ███ █████ ██████████████ ████ ██ █████████ █████ █████████
The question isn't which cities have healthier water, it's why the cities that use chlorine have more bacteria in their water. The health effects of chlorine don't help to explain that result.
When municipalities decide ███ ██ ██████████ █████ █████ █████████ ██ ██ ███████ ███████ █████ ████████ ████ ██████ ██████████ ██ ███ █████ ███ █████ ██ █████████
We're not asking why the different cities decided whether or not to use chlorine, we're asking why the ones that did use chlorine still had more bacteria in their water. The initial reasons for making that decision don't help us unless they explain the unexpected bacteria levels.
The municipalities that ███ ███ ███ ████████ ██ █████ █████ ████████ ████ ███ ███ ███ ███ ██ ███ █████ █████████ █████ ██████████████ █████ ███ ████ █████████ ████ █████████
(C) only makes the result more unexpected. If the non-chlorinating cities did use another disinfectant, that would explain their lower bacteria levels. But as it is, (C) just eliminates a possible explanation.
Other agents commonly █████ ██ ██████ █████ █████████ ████ ██ ████████ ███ ██████ ██████████ ████ ███ ████ ██ ███ ██ ███ ██ ███████████████
Much like (C), (D) eliminates a potential explanation. If non-chlorinating cities used some other disinfecting agent, that could explain the result. But if none of the cities used anything other than chlorine, we're still confused.
Municipalities that do ███ ██████████ █████ █████ ████████ ███ ███████ ██ ████████ ██████████ ██ ███ ████████ ██████████ ██ ██████ ██ █████ ███ █████ █████ ████ ███ ██████████████ ████ ███ █████████
(E) gives us the relevant difference in pipes and storage tanks that we're looking for. We know that the water isn't contaminated to begin with, but that pipes and storage tanks can cause contamination. If non-chlorinating cities simply avoid these sources of contamination (due to stricter regulation), that tells us why they have lower bacteria levels.