Which of these is a disadvantage of hydrogen fuel cells? a. engines that use hydrogen fuel are more efficient than gasoline-powered engines. b. the only emission produced by hydrogen fuel cells is water. c. hydrogen fuel is continually available and useful for heating. d. hydrogen fuel is often purified in a process that emits lots of greenhouse gases.

Hydrogen fuel is often purified in a process that emits lots of greenhouse gases. (D)

Explanation: Hydrogen is a fuel used in fuel cells and the only two things that you need for the reaction to occur is hydrogen and oxygen.  

The disadvantages are that it requires a special engine to run, and that the engine is expensive and must be kept safe since hydrogen gas is highly flammable and highly explosive.

Moreover, it contributes to global warming as using hydrogen as a fuel not only comes from water and oil but also comes from fossil fuels and can make the global warming worse.