I think there is another flaw with B. Even if you change "many" to "most" I don't see that as impacting the argument. Even if most marketing campaigns fail, what drawback is there to trying it anyway? B doesn't give us any reason NOT to try to marketing campaign, because there is no cost to simply trying, even if you were likely to fail. D on the other hand reveals that there is indeed a cost to attempting the marketing campaign in the first place.

For E, what about the fact that the passage doesn't explicitly specify that an antibiotic is or has been used against X? I was thinking E was wrong because the passage doesn't specify that antibiotics were ever used on X before

"Solely" really screwed me up. In retrospect though, the solely is really needed for this to be a proper SA question. Since the rule ensures that the stimulus must end up being true, the solely needs to be there so that the stimulus can conclude beyond reasonable doubt that Checkers was trying to hurt Marty's.