(a) Hydrogen causes minimum environmental pollution. It is the lightest and most abundant chemical element. Naturally occurring hydrogen is relatively rare on the earth. Hydrogen fuel is a zero-emission fuel when burned with oxygen. It can be used in electrochemical cells or internal combustion engines to power the vehicles. It has been started to be used in commercial fuel cell vehicles such as in passenger cars and buses. It is also used as a fuel for the propulsion of spacecraft.