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.
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.
(B) The pollution may have been reduced to a level at which the salmon can survive.
(C) Oxygen starvation is often a consequence of pollution, and this may have killed the salmon.
(D) The salmon may have been killed by one particular pollutant, which has now been removed while others remain.
(E) There may still be pollution, but its nature may have changed to a form that salmon can tolerate.
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