A great answer from pqhai to a great question. I would only add my few points in it.
Two types of earthworm, one black and one red-brown, inhabit the woods near the town of Millerton. Because the red-brown worm's coloring affords it better camouflage from predatory birds, its population in 1980 was approximately five times that of the black worm. In 1990, a factory was built in Millerton and emissions from the factory blackened much of the woods. The population of black earthworms is now almost equal to that of the red-brown earthworm, a result, say local ecologists, solely stemming from the blackening of the woods.
Which of the following, if true, would most strengthen the conclusion of the local ecologists?
A]The number of red-brown earthworms in the Millerton woods has steadily dropped since the factory began operations.
B]The birds that prey on earthworms prefer black worms to red-brown worms.
C]Climate conditions since 1990 have been more favorable to the survival of the red-brown worm than to the black worm.
D] The average life span of the earthworms has remained the same since the factory began operations.
E]Since the factory took steps to reduce emissions six months ago, there has been a slight increase in the earthworm population.
What is wrong with option
As we have to prove that after factory operations the red ones were more visible than the black ones ,so this should provide support to "Black = Red solely stemming from the" blackening of the woods."
Knowing that factory operations was the cause of " blackening of the woods."
IMO, this question though worded as strengthen question is actually testing whether we can figure out the defender assumptions the author has made while drawing the conclusion.Brief about Defender Assumptions
In general there are two types of assumptions.
1) Supporter Assumption :- This types of assumptions are common and plays traditional linking role - connecting the pieces of the arguments to one another by usually providing new information and filling out the gap between premises and conclusion.
2) Defender Assumptions :- These are of altogether different type. They protect the argument by eliminating the ideas that could weaken the argument.
Consider the following argument :- People who read a lot are more intelligent than other people. Thus, reading must cause a person to be intelligent.
Following are some of the statements that could attack the above conclusion
1) Regular exercise causes a person to be intelligent.
2) A high-protein diet causes a person to be intelligent.
So to hold the conclusion true, author assumes that the above scenarios do not exist.
1) Regular exercise does not cause a person to be intelligent
2) A high-protein diet does not cause a person to be intelligent.
So we can infer that defender assumptions are basically eliminates the possibility of any alternate explanation for the given effect.
Back to the question...........
Premise 1 :- During 1980s population of red-worms was approx 5 times that of black worms, because the coloring of red-worms was matching with the surrounding environment (esp tree's woods) and protecting them from predators.
Premise 2 :- During 1990s emissions from newly built factory blackened much of the woods. This change took back the color match advantage that red-worms were getting till date.
Premise 3 :- At the same time population of red worms became almost equal to that of black worms.
Conclusion :- The decrease in the population of red-worms is due to the blackening of the woods.
Here the ecologist attributed the decrease of red-worms' population solely to the blackening of the woods caused by emission. So while drawing this conclusion he must have assumed that any alternate explanation for the decrease in population of red-worms does not exists.
Since we are told to strengthen the ecologist's conclusion, our job is to refute the possibility of any alternate explanation.
Choice A :- INCORRECT. Shell Game Answer.
This choice merely restates the premise. It does not add any new information that would affect the conclusion.
Choice B :- INCORRECT. This choice does not make any sense with the conclusion. If the predators had preferred black worms over red-worms then the population of red-worms certainly would have not dropped substantially.Choice C :- CORRECT. Changed environmental condition could also be the cause of decrease in red-worms' population. THIS CHOICE eliminates the possibility of existence of such scenario and makes the ecologist's conclusion valid.
Choice D :- INCORRECT. irrelevant
Choice E :- INCORRECT. irrelevant.
Articles Subject-Verb Agreement NEW!!
Practice Combinations and Probability | DS Combinations | PS Absolute Value and Modules | DS Absolute Value and Modules | Critical Combinatorics
Collection Critical Reasoning shortcuts and tips | OG Verbal Directory (OG 13 and OG Verbal 2)
Contributing to Each Other’s Learning!!!