Hi, the author sets out two possibilities, either CFS is caused by one virus or by several. They then introduce the evidence that an antiviral has helped with all the symptoms of CFS. From thus, the author concludes that it is likely one virus.
Theres a big issue here- the evidence just doesnt support the conclusion. I have no idea that one antiviral working on all the symptoms makes it more likely that it's only one virus causing it. It's entirely possible that one antiviral could combat several viruses.
B both corrects this issue, and is something the author must believe, so it is certainly a necessary assumption.
