Question

lightestnon lnflamable gas is?

Answer 1

Helium is the lightest and non flammable gas. Because hydrogen which is the lightest gas is highly flammable. So the second lightest gas, being helium is non flammable. Thus, helium is the answer.
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Answer 2

Helium is the second lightest gas. The only gas lighter than helium is hydrogen, which is flammable. I believe neon and argon are lighter than co2, but still much heavier than helium