Recent studies have indicated that a certain type of freshwater cod has more tumors than other species of fish in the Hudson River. Long before this phenomenon was recognized, significant progress had been made in clearing the river of chemicals and other kinds of pollution thought to promote tumor growth in fish.
Which one of the following conclusions can most reliably be drawn from the statements above?
A. There is no causal link between pollution and the tumors in the freshwater cod.
(A) is incorrect because it it precisely the opposite of what the author is implying. The author is claiming two things: (1) that the cod now have tumors; (2) that before the tumors were discovered, progress was made in cleaning up the river. Note that the stimulus tells us only that "progress had been made;" it does NOT tell us that the pollution had been eliminated entirely. Therefore, if the fish have tumors, it is reasonable to assume that there is still some pollution in the river, that the pollution caused the tumors, and that efforts to clean up that pollution didn't do any good.
B. Efforts to clear the Hudson of chemicals and other kinds of pollution have not been strong enough.
(B) is correct because it is the only reasonable conclusion one can draw from the two items of evidence cited above. The fish have tumors, despite the fact that they tried to clean up the river years ago before the tumors were discovered. The author is clearly implying that the tumors were caused by pollution. Note, once again, that the stimulus DOES NOT SAY that the pollution was COMPLETELY ELIMINATED. It only says that "progress had been made." The reasonable and obvious conclusion is that they did not do a good enough job cleaning up the river, because if the fish have tumors anyway then there must still be some pollution in the river. Whatever progress was made was not good enough.
C. A sudden change in the river environment has had a drastic effect on the freshwater cod.
(C) is incorrect because nothing in the stimulus leads to this conclusion. There is nothing "sudden" or "drastic" that can be inferred from the author's statements. The author is merely pointing out the existence of a condition and attempting to suggest its cause. Nothing in the stimulus suggests that the river "suddenly" became polluted, or that the tumors were caused by anything that "suddenly" changed.
D. No other fish besides the freshwater cod can be harmed by chemicals and other kinds of pollution.
(D) is incorrect because the distinction between the cod and other fish is not the author's point. There is nothing in the stimulus to indicate that he thinks other fish are immune to pollution. The stimulus states that the cod have "more tumors" than the other fish, not that the cod have tumors and the other fish do not.
E. The studies mentioned provide no evidence that the number of tumors in the freshwater cod is related to the amount of pollution in the river.
(E) is incorrect because it says essentially the same thing (A) does, except this time the statement is attributed to "the studies mentioned" instead of being claimed outright. This might be the correct answer if the question was which statement would weaken the argument, or which statement could be used to refute or rebut the argument, but that is not the question. The question asks for the most likely conclusion. A conclusion should be a statement of what the author seems to believe, i.e., where his statements logically lead. The stimulus cannot reasonably be read to suggest that the author believes that pollution was NOT the cause of the tumors. In addition, the statement in (E) as constructed simply does not work as a conclusion. It would not make sense, for example, to add the word "Therefore, ..." to the beginning of the sentence.