I can't grasp why B weakens this argument and C doesn't. The only scenario I can think of is if a symptom of a disease is required for that disease, but can't a disease have multiple symptoms that aren't always present? Just because not all victims of ebola aren't afflicted with hiccups doesn't mean that many aren't. And also the questions stem itself says hiccups were experienced by "many", not all... If all victims of ebola were afflicted, then yeah that would definitely weaken it, but I don't see how it does not.
As for C, I don't see how that could weaken it less than B, since there are plenty of ways someone could have been infected outside of athens and brought the disease back to the city....
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B does not weaken the argument because we don't need to know that ALL people with Ebola get the hiccups. What if it is the case that of the hypothetical 1 million people who have died from Ebola, 999,999 have developed the hiccups? Then we have really good reason to think that hiccups and Ebola are linked, even though not all of the victims of Ebola got hiccups. So B does not weaken.