It might help to know what put you off B and on to C. Others have covered B quite well. For C, I'd say that we don't have any specific numbers here at all. We don't know the cost of the library, the size of the fee, etc. So even if we were told that, say, 10,000 people visited the beach last year, we wouldn't know what to do with that information. In any case, we've been told that the fee is enough to cover the costs if people continue to visit at current levels, whatever those are. So we don't need to know anything about the actual number--we just need to know whether it will meet/exceed current levels or drop, and that's what B addresses.
In general, be very wary of answers that ask for specific numbers. Often, that kind of information wouldn't help us to evaluate the argument unless we had a lot of other specific information, as well. Remember that we trust the premises. If they say current levels will do, then current levels will do.
Rege1988
Can somebody explain to me why is the answer is B and not C?