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
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
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
- To avoid manatee-boat collisions, boat traffic in manatee-populated waters is being required to maintain very low speeds.
- Manatees are unable to hear low-pitched sounds.
- A boat’s sound lowers in pitch as the boat slows.
This approach (the approach is 'reducing boat speeds') may make things worse.
If we have to cast doubt on the conclusion, we have to show that reducing boat speeds may not make things worse.
Option (A): It doesn't matter how the areas were decided. This option doesn't say that reducing boat speed will not increase collisions.
Option (B): Out of scope of our question. We need to focus on boat-manatee collisions.
Option (C): Boat traffic is increasing. But it doesn't say how reducing speed may not increase number of collisions.
Option (D): It does have something to do with speed of sound. But it says that sound of a boat travels much further under water than through air. If sounds are low pitched manatees may not be able to hear them, irrespective of how far the sounds travel and how far the manatees are from the boat. Lowering of speed of boats could make the situation worse and hence, it doesn't weaken my conclusion.
Option (E): Manatees are unable to discern sounds of boat moving at different speeds over background noise. This means, it doesn't matter at what speed the boat is moving. The manatee will not be able to discern its sound (whether high pitched or low pitched). Hence, reducing the speeds of the boats will not have a negative impact on the number of collisions. It might even have some positive effect since the slow moving boat may be able to see the manatee and change its course in time. Hence, this option weakened our conclusion.
Note: To weaken the conclusion, I only need to prove that the approach may not make things worse. I don't have to show that it necessarily improves matters.
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