The central idea that organizes the essay is "causes and effects of building river dams", so these two are the major points that the essay needs to talk about, other points may be included only if they are relevant to the topic. Thus, since Jaycee already wrote about the reasons that river dams are built (The causes), the next paragraph should focus on the effects of river dams.
Options B, C, and D are incorrect because they all express points that can be included in the paragraph that focuses on the reasons why river dams are built.
