Which of the following is the largest disadvantage of hydropower

Hydro power, although a low-cost option, is not implemented every time because of the amount of disadvantages there are. One of the disadvantages this has is the pollution is gives. Since the reservoir is always filled with water, this causes any surrounding plants and trees to decompose, releasing a huge amount of methane and carbon dioxide into the environment. Aside from this, it also destroys nearby habitats, and diverts natural waterways, effectively killing fishes too.

Edit the post so i can see the letters for each answer, and increase the points required so i'd be able to answer it.