Critical Reasoning

GmatGreen wrote:Mayor: Migrating shorebirds stop at our beach just to feed on horseshoe-crab eggs, a phenomenon that attracts tourists. To bring more tourists, the town council plans to undertake a beach reclamation project to double the area available to crabs for nesting.

Birdwatcher: Without a high density of crabs on a beach, migrating shorebirds will go hungry because shorebirds only eat eggs that a crab happens to uncover when it is digging its own nest.

Which of the following, if true, would provide the mayor with the strongest counter to the birdwatcher's objection?

(A) Every year a certain percentage of crabs are caught by fishermen as bait for eel traps.

(B) Horseshoe crabs are so prolific that given favorable circumstances their numbers increase rapidly.

(C) On average, tourists who come to the town in order to watch birds spend more money there than tourists who come for other purposes.

(D) The additional land made available by the reclamation project will give migrating shorebirds more space.

(E) Some of the migrating shorebirds make only one stop during their migration from South America to Canada.

So, we want to WEAKEN the birdwatcher's argument.

SUMMARIES
Mayor: Tourists come to watch birds eat crab eggs. So, let's double the size of the crab nesting area.
Birdwatcher: The crab eggs that the birds eat are uncovered by crabs building their nests. So, the decreased density of crab eggs will cause the birds to go hungry.

The idea here is that, when the crab-nesting area is relatively small, the density of crab eggs (in the sand) is quite high, so nesting crabs are quite likely to uncover eggs, so the birds have lots to eat. So, if crab-nesting area is doubled, the density of crab eggs won't be as high, and nesting crabs will be LESS LIKELY to uncover eggs, which means there will be FEWER eggs for the birds to eat.

So, the birdwatcher's conclusion is If the crab-nesting area is doubled, the birds will go hungry.

As we examine each answer choice, we'll remind ourselves of the birdwatcher's conclusion by asking, Does this premise weaken the conclusion that if the crab-nesting area is doubled, the birds will go hungry?

(A)Every year a certain percentage of crabs are caught by fishermen as bait for eel traps.
Does this premise weaken the conclusion that if the crab-nesting area is doubled, the birds will go hungry?
No, it has no effect. ELIMINATE A

(B) Horseshoe crabs are so prolific that given favorable circumstances their numbers increase rapidly.
Does this premise weaken the conclusion that if the crab-nesting area is doubled, the birds will go hungry?
YES. This premise suggests that, if the nesting area is increased, the crab population will increase. The birdwatcher's argument that the density of crab eggs (in the sand) would decrease if the nesting area were doubled was based on the ASSUMPTION that the crab population would not increase. Answer choice B suggests that the crab population would increase, which means the density of crab eggs (in the sand) might not decrease, which means nesting crabs may continue to uncover lots of eggs for the birds to eat.
KEEP B for now.

(C) On average, tourists who come to the town in order to watch birds spend more money there than tourists who come for other purposes.
Does this premise weaken the conclusion that if the crab-nesting area is doubled, the birds will go hungry?
No, it has no effect. ELIMINATE C

(D) The additional land made available by the reclamation project will give migrating shorebirds more space.
Does this premise weaken the conclusion that if the crab-nesting area is doubled, the birds will go hungry?
No, it has no effect. ELIMINATE D

(E) Some of the migrating shorebirds make only one stop during their migration from South America to Canada.
Does this premise weaken the conclusion that if the crab-nesting area is doubled, the birds will go hungry?
No, it has no effect. ELIMINATE E

So, the correct answer must be B

Cheers,
Brent