Answer:
A. Preventing fish from moving along a river.
Step-by-step explanation:
1. Dams create barriers and trap the water that builds up behind it. This prevents fish from moving along the river as there's a barrier in place and is just overall harder for the fish to move freely along a river because of it.
2. It's the only negative option as the others are all positive outcomes, while the question asks for a negative outcome instead.
Hope this helps!