Concept:An inert gas surrounds the tungsten filament and slows its vaporization.
Explanation:The filament in a light bulb is made of tungsten, which tends to vaporize at high temperatures.
Argon is a heavy, inert gas that does not react readily with the filament.
It creates a protective atmosphere around the filament.
This atmosphere reduces the rate at which tungsten atoms escape by vaporization.
Since argon is denser than lighter inert gases, it retards vaporization more effectively.
Thus, argon helps increase the lifespan of the light bulb.
Answer:A. Argon