Manatees, aquatic mammals inhabiting Florida’s rivers and

[Khush]

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.

OA: E
why is E correct? doesn't E say that manatees can't make out low-pitched sounds? i rejected it as i thought E strengthens the argument.

D says sound travels more distance in water than in air. doesn't that say manatees can make out noise better?

this is a tough question. need help here.

[RonPurewal]

why is E correct? doesn't E say that manatees can't make out low-pitched sounds? i rejected it as i thought E strengthens the argument.

E says that the manatees were unable to hear the boat noises at any speed that was tested ("various speeds"). In other words, they are already deaf to the boat sounds, so there is no way that things could get worse at lower speeds.

[RonPurewal]

D says sound travels more distance in water than in air. doesn't that say manatees can make out noise better?

If a sound has a pitch that they can't hear, then, even if that sound travels a greater distance, they still can't hear it.

Just think about your own hearing. You can't hear dog whistles. If someone made a dog whistle that projected its sound over 100 miles ... well, you still wouldn't be able to hear it.

[Khush]

Makes sense, Ron!
Thank You!!

[RonPurewal]

Sure.

[qianruS779]

Please explain why the choice B is not correct. I understand that Manatees can benefit in water with slow boat speed, so this approach does not make things worse.

Thank you very much.

[RonPurewal]

Please explain why the choice B is not correct. I understand that Manatees can benefit in water with slow boat speed, so this approach does not make things worse.

Thank you very much.

remember to pay attention to the specific requirements of the question.

here, you are asked to 'cast doubt on the conclusion'.
in this argument, 'the conclusion' is about how the number of manatees killed in collisions with boats might be affected by different boat speeds.
the problem with choice B is that it has no bearing on this reasoning.

in other words:
• you're supposed to show that the conclusion itself might be wrong (= that slower boats might hit manatees just as often as faster ones).
• choice B doesn't do that. choice B is more like 'okay, that reasoning is totally valid, BUT here's another factor that might concurrently work the other way'.

[RonPurewal]

also, the other problem with B is that it talks only about 'FAST-moving boat traffic'... and we don't know whether this is relevant.

the current changes require boats to move at 'very low speeds'.

...so we know that, BEFORE these changes, boats moved at some speed that was not 'very low'.
that could have been 'fast'... but it also could have been 'normal' or even 'somewhat low'.

in other words:
Before, boats moved at ____ speeds. Now, they must move at VERY low speeds.
to consider choice B relevant you have to assume—at random—that '___' is 'FAST'. there's no good reason to make that assumption.

[qianruS779]

Hi Ron,

Thank you very much for your detailed explanation, and I got your point.

Best, Song

[RonPurewal]

you're welcome.