Just a few years ago salmon could not survive in the oxygen-starved an

C is the right answer.


We are dealing with a cause and effect argument, and this cause and effect relationship is expressed within the conclusion of the argument. Cause: the salmon have returned. Effect: A sure sign that the river is pollution-free. Per the way the conclusion is drawn, we need an answer choice that provides an alternative reason for the return of the salmon in the Thames, suggesting that although the salmon have returned, it is not because the Thames is no longer polluted.


(A) The salmon that have returned may be of a strain that is unaffected by the pollutants.
This is exactly in line with the reasoning above. So the salmon that are now in the Thames may be of a new strain that can withstand the current pollutants in the river. Not our answer.

(B) The pollution may have been reduced to a level at which the salmon can survive.
The conclusion is categorical. The river is pollution-free. So, B states that the pollution may have reduced to a level at which the salmon can survive, implying that the water is still polluted. Hence the argument is weakened. Not our answer.

(C) Oxygen starvation is often a consequence of pollution, and this may have killed the salmon.
The conclusion drawn above is not dependent on oxygen starvation. So This answer choice is irrelevant, and hence it does not weaken the argument above. This is our answer.

(D) The salmon may have been killed by one particular pollutant, which has now been removed while others remain.
Well, as stated in B, the conclusion of the argument is categorical. The return of the salmon is a sure sign that the river is pollution-free. Now, C is telling us that the Salmon have returned because a particular type of pollutant that has now been removed was responsible for the death of the salmon. Hence now that that particular pollution has been gotten rid of, the river still has some other pollutants in it, hence not pollution-free. This option weakens the argument and can thus be discarded.

(E) There may still be pollution, but its nature may have changed to a form that salmon can tolerate.
This option also suggests that the river is not pollution-free after all. It, therefore, weakens the argument, hence not our answer.

Just a few years ago salmon could not survive in the oxygen-starved and polluted Thames. Nor could many other species. But now, after years of determined effort, the salmon have returned, and that is a sure sign that the river is pollution-free.

Stimulus: The stimulus states that due to pollution the salmon could not survive in the Thames. After years of effort the salmon have returned which means the river is pollution free.

Each of the following indicates a possible flaw in the reasoning in the passage above EXCEPT:


(A) The salmon that have returned may be of a strain that is unaffected by the pollutants. This shows a flaw in reasoning stating that the Thames is still polluted

(B) The pollution may have been reduced to a level at which the salmon can survive. Even though the pollution levels are reduced. The stimulus says Thames is pollution free

(C) Oxygen starvation is often a consequence of pollution, and this may have killed the salmon. Agrees with stimulus IMO C

(D) The salmon may have been killed by one particular pollutant, which has now been removed while others remain. This gain refutes the authors point of pollution free Thames

(E) There may still be pollution, but its nature may have changed to a form that salmon can tolerate.Pollution exists