What Roles Do Hydrogen Gas and Carbon Dioxide Play in the Metabolism

What roles do hydrogen gas and carbon dioxide play in the metabolism of methanogens?

A) Hydrogen gas is their energy source, and carbon dioxide is their terminal electron acceptor.
B) Hydrogen gas is their terminal electron acceptor, and carbon dioxide is their energy source.
C) Hydrogen gas is their energy source, and carbon dioxide is their carbohydrate source.
D) Hydrogen gas is their initial electron acceptor, and carbon dioxide is their energy source.
E) They use hydrogen gas in respiration, and they use carbon dioxide in photosynthesis.