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?
A: Until recently, treatment X was more expensive than treatment Y.
B: Unlike treatment Y, treatment X has produced harmful side effects in laboratory animals.
C: Treatment Y is prescribed more often by physicians than treatment X.
D: A third treatment, treatment Z, is even quicker and less expensive than treatment X.
Choose A, B, C or D as your solution.
B