The correct answer is - B) Building a dam will disrupt the local ecosystem.
The dams are very useful, as they produce very small amounts of pollution, create a lot of electric energy, provide water for irrigation, even for drinking, create nice recreational spots.
But despite all the positives, there's a negative thing that comes along with them, and that is the disruption of the local ecosystem. By building a dam the water temperature changes, or rather it increases because the water is more static. The organisms that live there are adapted to certain temperature of the water, so that immediately causes big problems for their survival. Also, the dam restricts the movement of the organisms, which is especially a big problem for the ones that migrate, be it for food or for mating, so it can cause the decline or even extinction of some species.