The organizers of tomorrow's outdoor concert announced that it will go on tomorrow on schedule unless bad weather is forecast or too few advance tickets are sold. If the concert is canceled, refunds will be made to ticket holders. Since some ticket holders have already been issued refunds even though more than enough advance tickets were sold, it must be the case that bad weather is forecast.
　　Which of the following is an error of reasoning con- tained in the argument?
　　(A) It proceeds as if a condition, which by itself is enough to guarantee a certain result, is the only condition under which that result would occur.
　　(B) It bases a conclusion that is known to require two conditions on evidence that bears on only one of those conditions.
　　(C) It explains one event as being caused by another event, even though both events must actually have been caused by some third, unidentified event.
　　(D) It treats evidence for the absence of one condi-tion under which a circumstance would occur as conclusive evidence that that circumstance will not occur.
　　(E) Evidence given to support the conclusion actually undermines it.

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