The lightest gas which is non-inflammable is A. Hydrogen B. Helium C. Oxygen D. Nitrogen

Hydrogen is the second lightest gas in the universe, and it is flammable. It is a colorless, odorless, and tasteless gas that is used in a variety of applications, including fuel cells, rockets, and airships.

Oxygen is the third lightest gas in the universe, and it is flammable. It is a colorless, odorless, and tasteless gas that is essential for life on Earth.

Nitrogen is the fourth lightest gas in the universe, and it is non-flammable. It is a colorless, odorless, and tasteless gas that is used in a variety of applications, including fertilizers, explosives, and refrigerants.