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I know others have told you the answer is B, but I'm going to explain why it is actually C.I believe it is obvious that dams hold back water so as to control flooding. Irrigation ditches (also known as irrigation canals) are intended to bring water from one place to another (hence, irrigate) for consumption of cities or other populated areas generally for crops. This helps control flooding because it takes away excessive water and brings it to a place that is usually experiencing a drought, but it is not the intended purpose, it is simply an added perk. A storm sewer (or storm drain) is similar to the irrigation ditches except its purpose is actually to prevent flooding. It is intended to carry away excess water in times of heavy rain or water related natural disasters. Walls and Levees can be natural or artificial but in either case it works almost as a dam and blocks water from going where is shouldn't, thus reducing flooding. Artificial ones were created for this sole purpose in mind. Through process of elimination, you can clearly see that the answer is Irrigation Ditches, which are not intended to reduce flooding.