Why is A correct?

I did not choose A because the cost/benefit analysis did not seem similar enough to be parallel. In A, part of the benefit seems to actually outweigh the cost that they were considering in the first place. In A, the benefit of exercise doesn't actually impact the cost (pain of surgery). In other words, it seemed to me that the stimulus was arguing that the cost wouldn't happen at all, since they would be able to cover the cost of the renovations with the extra income. A seemed really different since being able to exercise wouldn't solve the problem of having pain after surgery. Should I instead have looked more generally, and assumed that it is correct merely because it is a cost-benefit analysis? Or is there something else that I am missing? Thank you!!