Here's a problem to solve: A city wants to build a dam nearby to increase water supplies. Which question is most important to ask about the environmental effects of building the dam?

Among the 4 following options, which is the correct answer?

- A: How long will it take to build?
 
- B: How much concrete is needed?
 
- C: How will the lake be used for recreation?
 
- D: How will the dam affect fish populations?
 