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Manatees, aquatic mammals inhabiting Florida’s rivers and coastal waters, swim close to the surface and are frequently killed in collisions with boats. To address the problem, boat traffic in manatee-populated waters is being required to maintain very low speeds. Unfortunately, manatees are unable to hear low-pitched sounds and a boat’s sound lowers in pitch as the boat slows. Therefore, this approach may in fact make things worse rather than better.

Which of the following, if true, casts most doubt on the conclusion?

A. The areas where boats would have to maintain low speeds were decided partly on the basis of manatee-population estimates and partly from numbers of reported
collisions between manatees and boats.
B. Because the water hyacinth that manatees feed on grows best in water that is nearly still, water hyacinth beds can be disturbed or damaged by fast-moving boat
traffic.
C. Over the last several decades, boat traffic in Florida’s coastal waters has been increasing almost continuously and now represents the greatest threat to the
endangered manatee population.
D. The sound of a boat engine generally travels much further under water than it does through the air.
E. When experimenters exposed manatees to the recorded sounds of boats moving at various speeds, the creatures were unable to discern the sounds over normal
background noise.

Isn't answer choice E contradicting the premise "manatees are unable to hear low-pitched sounds and a boat’s sound lowers in pitch as the boat slows", the opposite of which will be that they are able to hear high pitched sounds? Or is the argument saying that they can't hear either?

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bounce87 wrote:
Manatees, aquatic mammals inhabiting Florida’s rivers and coastal waters, swim close to the surface and are frequently killed in collisions with boats. To address the problem, boat traffic in manatee-populated waters is being required to maintain very low speeds. Unfortunately, manatees are unable to hear low-pitched sounds and a boat’s sound lowers in pitch as the boat slows. Therefore, this approach may in fact make things worse rather than better.

Which of the following, if true, casts most doubt on the conclusion?

A. The areas where boats would have to maintain low speeds were decided partly on the basis of manatee-population estimates and partly from numbers of reported
collisions between manatees and boats.
B. Because the water hyacinth that manatees feed on grows best in water that is nearly still, water hyacinth beds can be disturbed or damaged by fast-moving boat
traffic.
C. Over the last several decades, boat traffic in Florida’s coastal waters has been increasing almost continuously and now represents the greatest threat to the
endangered manatee population.
D. The sound of a boat engine generally travels much further under water than it does through the air.
E. When experimenters exposed manatees to the recorded sounds of boats moving at various speeds, the creatures were unable to discern the sounds over normal
background noise.

Isn't answer choice E contradicting the premise "manatees are unable to hear low-pitched sounds and a boat’s sound lowers in pitch as the boat slows", the opposite of which will be that they are able to hear high pitched sounds? Or is the argument saying that they can't hear either?
It might be helpful to consider a simpler version of the logic.

Imagine there's an epidemic in a particular neighborhood of small children getting hit by cars. The proposed solution: lower the speed limit. Now someone in the neighborhood insists that this solution will make the problem worse, as slower cars make less noise and so the kids won't hear the cars coming.

If it turned out that all the kids playing in the street had earbuds in and so couldn't hear the cars even when they were driving fast, then we know that the speed limit won't make things worse. The kids never heard the cars in the first place, and so the fact that there are quieter cars doesn't present a new problem.

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bounce87 wrote:
Manatees, aquatic mammals inhabiting Florida’s rivers and coastal waters, swim close to the surface and are frequently killed in collisions with boats. To address the problem, boat traffic in manatee-populated waters is being required to maintain very low speeds. Unfortunately, manatees are unable to hear low-pitched sounds and a boat’s sound lowers in pitch as the boat slows. Therefore, this approach may in fact make things worse rather than better.

Which of the following, if true, casts most doubt on the conclusion?

A. The areas where boats would have to maintain low speeds were decided partly on the basis of manatee-population estimates and partly from numbers of reported
collisions between manatees and boats.
B. Because the water hyacinth that manatees feed on grows best in water that is nearly still, water hyacinth beds can be disturbed or damaged by fast-moving boat
traffic.
C. Over the last several decades, boat traffic in Florida’s coastal waters has been increasing almost continuously and now represents the greatest threat to the
endangered manatee population.
D. The sound of a boat engine generally travels much further under water than it does through the air.
E. When experimenters exposed manatees to the recorded sounds of boats moving at various speeds, the creatures were unable to discern the sounds over normal background noise.
Premise: Manatees cannot hear the low-pitched sounds produced by a slow-moving boat.
Conclusion: Requiring boats to maintain very low speeds will make things worse for the manatee.

E: When experimenters exposed manatees to the recorded sounds of boats moving at various speeds, the creatures were unable to discern the sounds over normal background noise.
Here, the manatee cannot discern a boat's sounds REGARDLESS OF THE BOAT'S SPEED, weakening the conclusion that requiring boats to maintain very low speeds will make things worse for the manatee.

The correct answer is E.

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