Answer choice B

Answer choice B is incorrect because of one key word: small. Only a small amount of the city water results from underground water systems, which means that most of the city's water comes from other sources, possibly including reservoirs and streams. If most of the city's water comes from sources apart from the underground water system, than increasing the water supply of those very sources, which the argument says occurs, doesn't weaken the conclusion; if anything, the argument is strengthened. Answer Choice C is correct because it essentially renders the surplus of water in this circumstance irrelevant. By invoking that the city population is too great to be efficiently supported by the current water pumping capacity of the water company, it shifts the responsibility from the water supply to the infrastructure. Having a surplus of water is great if you have the ability to utilize it. If you don't, then you may need to water ration. Hope this helps!