This doesn't help. What's surprising in the stimulus isn't that "some" antibiotics work where others do not — we never compare any antibiotics with others. What we want to explain is why any antibiotics are successful in treating what appears to be an effect of a viral infection, when the stimulus tells us directly that antibiotics in general have no effect on this virus.

This explains the result. If the middle ear infection is not caused directly by the virus, but rather by an additional bacterial infection, this explains why antibiotics often work to clear up the middle ear infection. (Notice that the stimulus never says antibiotics work to clear up the viral infection). This suggests an alternate cause for the middle ear infection that is correlated with the viral infection.

This isn't useful. We're interested in why children who are infected with the virus and also develop middle ear infections are successfully treated for the ear infections by antibiotics. Children with middle ear infections, but not infected with the virus, aren't relevant to this question. They could have developed their ear infections for other reasons, while the stimulus implies that the middle ear infection in children who have the viral infection is correlated with, if not caused by, the viral infection.

This doesn't help. We're not interested in whether viral infections are harder to treat overall than bacterial infections. We want to explain why one specific form of treatment — antibiotics — which we are told has no effect on the virus in question, nevertheless successfully treats what appears to be an effect of this viral infection.

This doesn't help. At most, it strengthens the stimulus's suggestion that there is a positive correlation, if not a causal link, between the viral infection and middle ear infections. But that only underlines the question of why antibiotics, which don't affect this virus, often successfully treat middle ear infections associated with this viral infection.