Explanation:

• The four key elements of life are: Carbon, Hydrogen, Oxygen and Nitrogen.
•  Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen.
•  Hydrogen is most abundant element in the universe.
• Hydrogen is also a promising source of "clean" fuel on Earth.
• Oxygen is an important element that is needed by most life forms on Earth to survive.