Pure substances are made of the same material and have almost the same properties such as: Carbon, Iorn, Water, Sugar, Salt, and Nitrogen.
Without its atmosphere, Earth would be
bombarded by meteorites, similar to mars, relatively lifeless. Without the atmosphere objects can just enter the atmosphere and cause serious harm to us, but with the atmosphere it basically reduces damage and shrinks it to size. Without the atmosphere earth would be like mars since it doesn't have a atmosphere. Without the atmosphere earth would be relatively lifeless since without air, us humans cant breathe so we will not survive.
Answer:
The correct answer is option D.
Explanation:
A fuel cell is defined as the electrochemical cell which converts the chemical energy of a fuel (often used hydrogen) and an oxidizing agent (often used oxygen) into electrical energy via a pair of redox reactions.
The reactions which occur in hydrogen-oxygen fuel cell are:
At cathode:
At anode: 
As we can see that reaction of hydrogen gas with oxygen gas gives us the water as a product which can be used for other things and it also doesn't pollutes environment.