The inert gas which is substituted for nitrogen in the air used by deep sea divers for breathing, is ________ ?

A. Argon
B. Xenon
C. Helium
D. Krypton

Heliox is a mixture of oxygen and helium and is often used in the deep phase of a commercial deep dive to eliminate nitrogen narcosis. Heliair is a form of trimix that is easily blended from helium and air without using pure oxygen. It always has a 21:79 ratio of oxygen to nitrogen; the balance of the mix is helium.