A) That’s called a hydrogen car, and they exist already.
B) Never, the output is water. Then you just use enegy to split the water back into hydrogen and oxygen before burning it back into water. Just add energy from your preferred renewable source.
Annual petrol usage by cars: ~1.5 x 10^12 litres.
Amount of water on Earth’s surface: 1.386 x 10^21 litres.
Percentage of water used annually: 0.0000001%
Number of years to deplete all water on earth’s surface: 1 x 10^9 years, aka 1 billion years.
Minus a few million if we’re driving yo momma around.
By far most of that water is not fresh water though
The conversion of saline into fresh water via the water cycle will be sufficient to keep the cars running, though maybe not humans. The proof is obvious and left as an exercise to the reader.
=)
