One problem with fuel-cell cars is that gas is not readily available in
pure form.
Answer here

Hydrogen.

Explanation: One problem with fuel-cell cars is that hydrogen gas is not readily available in pure form. Fuel cell vehicles use hydrogen gas to power an electric motor. Unlike conventional vehicles which run on gasoline or diesel, fuel cell cars and trucks combine hydrogen and oxygen to produce electricity, which runs a motor. Hope this helps ^-^.

c u c l 2 + 2 n a n o 3 → c u ( n o 3 ) 2 + 2 n a c l

  long weird answer but i hope this works

the answer to this question is 3.