Correct Response: B. The most commonly used fuel cells chemically combine hydrogen with oxygen. Instead of producing only thermal energy, which is generated when hydrogen undergoes combustion with oxygen, the reaction is modified to produce mostly electrical energy. The electrical energy can then be used to perform useful work. The combination of hydrogen and oxygen also produces water (H2O), which is an output of this process. Hydrogen (A) is an input for a fuel cell. Carbon dioxide (C) is an output for the combustion of carbon in oxygen, not hydrogen, and is produced whenever fossil fuels are used. Methane (D) is a hydrocarbon gas that is the primary component of natural gas, and it is often used as an input for internal combustion engines.
