cars powered by hydrogen fuel cells emit which substance?\noxygen\ncarbon dioxide\nhydrogen\nwater

Brief Explanations:

In hydrogen - fuel - cell cars, hydrogen reacts with oxygen in the fuel cell. The chemical reaction produces water as the main product. There is no carbon - containing fuel burned, so no carbon dioxide is emitted. Hydrogen is the fuel, not the emission. And oxygen is consumed rather than emitted.

Answer:

D. water