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#19 - When permits for the discharge of chemicals into a

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Can someone help me understand how I could choose A over B? I understand B's flaws - I was hesitant to choose it because of the answer stated, 'there is a swift flow' when in fact we don't know if there is a swift flow or that there has to be a 'swift' flow - just that the flow amount affects the dilution. I thought the faster it flows the more the water is diluted, thus lessening environmental impact. For some reason, I translated 'amount' into speed. But, even considering this, answer B looked off because of the assertion that there absolutely is swift flow. But I chose it anyways because I was thinking, who cares about the various chemicals stated in Answer A? The stimulus only states that different chemicals have different dispersion requirements. In retrospect, I can see why A could be the correct answer, but I just sped past it thinking different chemicals interacting was irrelevant.
Robert Carroll
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I'll start with answer choice (B). There is no need to assume a "swift flow" because the stimulus already claims that the permitted amount is based on "an estimate of the effect of the dilution of the chemical by the amount of water flowing through the waterway." Thus, if the flow were not swift, that slow flow would be accounted for by the estimate. If that caused the chemical to become diluted at a slower rate, thus possibly polluting the waterway worse, then the permitted amount would be reduced - because it's based on an estimation of dilution that includes the actual amount of water flowing. If that water flows slowly, swiftly, or anything in between, the estimate is taking that into account. Thus, you don't need to assume any specific rate of flow. This is why answer choice (B) is not something that must be assumed.

Answer choice (A) is correct because it addresses a potential weakness in the argument. Think about it - you get a permit for each chemical based on a separate calculation for each chemical of the effect of dilution by the water flow. I'll use an example. Imagine that in waterway W, the amount of water flowing dilutes chemical X to such an extent that discharging 100 pounds of it per day is acceptable. Also in waterway W, the amount of water flowing dilutes chemical Y to such an extent that discharging 50 pounds of it per day is acceptable. The separate calculations for each chemical would tell me that a company would get a permit for discharging up to 100 pounds of chemical X per day, and a permit for discharging up to 50 pounds of chemical Y per day. What if chemical X and chemical Y interact so that they're more dangerous together - what if they react to form chemical Z, which is not diluted by the flow of waterway W at a sufficient rate to render it harmless? Then perhaps 100 pounds of chemical X (which the calculated estimate says is fine) and 50 pounds of chemical Y (again, fine by the estimate) cannot safely be discharged together in waterway W.

Answer choice (A) addresses this weakness and negates it. Thus, it's a Defender Assumption of the stimulus.

Robert Carroll