Logical Reasoning

PT16 · S2 · Q4 Although water in deep aquifers

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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.

Given that ...

Chlorine is often added to water as a disinfectant to protect against water getting contaminated as a result of flaws in pipes or storage tanks.

Thirty municipalities chlorinated their water; 20 did not.

How can it be that ...

The 20 that did not chlorinate had less bacterial contamination than the 30 that did.

Evaluate

Chlorination kills bacteria, so chlorinated water should be cleaner. Why are the chlorinated water cities doing worse, when it comes to contamination?

This is similar to other anti-causal setups: - how come the people who wear the most sunscreen get the most sunburns? - how come the students with the most extra help time from the teacher get the worst grades?

Does sunscreen / extra teacher time / chlorine just not do what it's supposed to do?

No, the way to resolve this paradox is usually to say that the people using sunscreen are the ones most vulnerable to sunburn in the first place. The students getting extra teacher help are the ones with the worst grades in the first place.

So the cities chlorinating their water might be the cities with the most contaminating pipes and storage tanks, in the first place.

Goal

Look for some way to explain why the chlorinated cities have more contamination. It will almost certainly provide some distinction between the 30 and the 20.

4.

Which one of the following, if true, most helps explain the difference in bacterial contamination in the two groups of municipalities?

  1. Chlorine is considered by some

    No Impact

    This doesn't offer any difference between the 30 chlorinating and 20 non-chlorinating cities, so it isn't going to help us explain the contamination difference between the 30 and the 20.

    3% picked this

  2. When municipalities decide not to

    No Impact

    This provides somewhat of a distinction. It suggests that in 30 cities, the citizens didn't object to the taste/smell of chlorinated water, whereas in 20 cities they did. But that doesn't offer any reason for why the water would be more contaminated in the cities where they disinfected the water with chlorine.

    1% picked this

  3. The municipalities that did not

    Cheats Paradox

    This makes us even more confused how the non-chlorinating cities have the less contaminated water. Not only did these municipalities not disinfect with chlorine, they also did not use any other disinfectant. So then how come they have less contaminated water than the cities that disinfected with chlorine?

    5% picked this

  4. Other agents commonly added to

    No Impact

    This fact is true for both the 30 chlorinators and the 20 non-chlorinators. Both groups equally lack those agents, so they cannot explain the difference in contamination.

    2% picked this

  5. Correct

    Municipalities that do not chlorinate

    Why this is right

    This offers a distinction between the 30 and the 20, telling us that the 20 have stricter regulations with their pipes and water tanks. In other words, they're forced to keep their pipes and water tanks in better shape. The stimulus told us that contamination can occur "as a result of flaws in pipes or storage tanks". Cities with tighter regulation will be less likely to have flaws, so less likely to be contaminated in the first place. That's why the non-chlorinators end up better. They prevent contamination at its source, whereas the other 30 cities are trying to de-contaminate dirtier water using chlorine (and it isn't getting rid of all the contamination).

    Skill tested: Paradox · how this choice captures the argument's function is the move to repeat next time.

    90% picked this

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