A researcher lacked the necessary equipment to measure methane production so instead monitored CO₂ concentration as an unknown archaeon grew and produced methane. Why might CO₂ NOT decrease while methane would still increase?

A) An alternative carbon source such as methanol was used.
B) CO₂ is not a carbon source used by methanogens.
C) CO₂ was used as an electron donor but not as a carbon substrate.
D) Methanogenic Archaea containing carboxysomes likely made measuring small losses of CO₂ difficult to conclude.