PT 60 S3 Q4

In a study of patients who enrolled at a sleep clinicbecause of insomnia, ... ...

Which one of the following, if true, most seriously weakens the argument above?
(A) Several studies have shown that inhaling the scent of peppermint tends to have a relaxing effect on people who do not suffer from insomnia.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Complete Section: LR-B, June '10 LSAT (PT 60 Sec. 3)