(A) gives information about reviewers of 1930s musicals, while Bordwell’s argument in the beginning of P3 is about viewers. Bordwell’s argument doesn’t mention reviewers, so this information isn’t relevant to the argument (and thus doesn’t weaken it).

Bordwell’s argument in the beginning of P3 concludes that audiences accepted the musicals as realistic. Knowing that the audiences enjoyed the musical performances doesn’t tell us anything about if the audiences saw the films as realistic overall.

This weakens Bordwell’s argument because his argument hinges on viewers’ knowledge of conventions of the genre. According to Bordwell, it is the conventions of musicals that prepare viewers to expect a different structure. However, in order to be prepared for these differences, viewers must be familiar with the conventions (and thus have been previously exposed to musicals). If it’s true that viewers experienced musicals in the same way whether or not they’d previously seen musicals, Bordwell’s argument is weakened because we can no longer say that the conventions of the genre prepare viewers for the different structure. (Remember, if it really is the conventions of the genre that cause viewers to experience a film a certain way, then those unfamiliar with the conventions would experience the film a different way than those familiar with the conventions.)

Bordwell’s argument in the beginning of P3 is about whether or not viewers perceived the 1930s musicals as realistic; this information about attention spans is not relevant to the argument.

This supports Bordwell’s claim that musical films evolved from live theater; this doesn’t weaken Bordwell’s argument.