Answer:
D. The North and South fought the Civil War for many reasons.
Step-by-step explanation:
This is the thesis statement that would be too broad for a short research report. In order to write a short research report, we need to be aware of the limited number of topic we can touch on. Therefore, if we are going to talk about the Civil War, we would need to limit our topics to two or three. By saying that there were "many reasons," we are being too vague and therefore, too broad on our thesis statement.