Why is B strong enough to weaken?

B was my first gut reaction and matched by anticipation; however, the "some computer brands but not others" part made it really suspicious to me because it seemed too weak and could apply to anything but not the two at hand. So I went with A because maybe it indicated that ProBit isn't that much more reliable because it is clustered with a bunch of others (think the more reliable part in the conclusion was an overall more, than a comparative more). I still didn't like A either. Any tips on weaken questions --- I feel like too often I pick too weak of an answers so I've been careful about strength, but clearly there are times where it can be pretty weak but still weaken overall...