CONCLUSION : The refinery is not to blame
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(A) The study did not measure the quantity of pollutants emitted into the surrounding air by the aluminum refinery.
All the statements in the arguments must be imagined to be true.we need to imagine that study included all the cases. So, eliminate it
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(B) The neighboring landholders have made no change in the way they take care of their plants.
out of scope -eliminate it.
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(C) Air pollution from the refinery has changed the chemical balance in the plants’ environment, allowing the harmful insects and fungi to thrive.
Its the weakener.Keep it.
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(D) Pollutants that are invisible and odorless are emitted into the surrounding air by the refinery.
same as A -Its tempting but company has already mentioed that its not because of pollution but because of insects and .... so eliminate it
(E) The various species of insects and fungi mentioned in the study have been occasionally found in the locality during the past hundred years. No need of past data to weaken the conclusion.irrelevant -eliminate it
C is the answer.!