Question

energy. these cells are expensive, but they produce no air pollution. what is the major by - product of hydrogen fuel cells? carbon dioxide water

Explanation:

Hydrogen fuel cells combine hydrogen and oxygen. The chemical reaction in them produces energy. The by - product of this reaction is water ($\ce{H2O}$). Since it is stated that these cells produce no air pollution, carbon dioxide (which is a greenhouse gas and can contribute to pollution in some contexts) is not the by - product. Water is a clean and non - polluting by - product of hydrogen fuel cell operation.

Answer:

Water