The inert gas which is substituted for nitrogen in the air used by deep sea divers for breathing is A. Neon B. Krypton C. Argon D. Helium

The correct answer is D. Helium.

Helium is the most abundant element in the universe, but it is only a very small fraction of the air we breathe. It is a colorless, odorless, and tasteless gas that is lighter

than air. Helium is used in deep sea diving because it is less soluble in blood than nitrogen. This means that divers can breathe helium-oxygen mixtures without the risk of nitrogen narcosis, a condition that can cause confusion, impaired judgment, and even death.

Neon, argon, and krypton are also inert gases, but they are not used in deep sea diving because they are more soluble in blood than helium. This means that divers who breathe these gases are at risk of nitrogen narcosis.

Helium is also used in balloons, blimps, and dirigibles because it is lighter than air. It is also used in cryogenics, lasers, and MRI machines.