Chronic fatigue syndrome is characterized by prolonged fatigue, muscular pain, and neurological problems. It is not known whether these symptoms are all caused by a single virus or whether each symptom is the result of a separate viral infection. A newly synthesized drug has been tested on those who suffer from chronic fatigue syndrome. Although the specific antiviral effects of this drug are unknown, it has lessened the severity of all of the symptoms of chronic fatigue syndrome. Thus there is evidence that chronic fatigue syndrome is, in fact, caused by one virus. The argument assumes which one of the following? ['An antiviral medication that eliminates the most severe symptoms of chronic fatigue syndrome thereby cures chronic fatigue syndrome.', 'It is more likely that the new drug counteracts one virus than that it counteracts several viruses.', 'Most syndromes that are characterized by related symptoms are each caused by a single viral infection.', 'All those who suffer from prolonged fatigue also suffer from neurological problems.']