Studies have shown that treating certain illnesses with treatment X produces the same beneficial changes in patients' conditions as treating the same illnesses with treatment Y. Furthermore, treatment X is quicker and less expensive than treatment Y. Thus, in treating these illnesses, treatment X should be preferred to treatment Y. Which one of the following, if true, would most weaken the argument above? ['Until recently, treatment X was more expensive than treatment Y.', 'Unlike treatment Y, treatment X has produced harmful side effects in laboratory animals.', 'Treatment Y is prescribed more often by physicians than treatment X.', 'A third treatment, treatment Z, is even quicker and less expensive than treatment X.']