After long periods of use, a gray spot develops on the inside of a bulb. This is because

(a) The tungsten filament evaporates and collects there

(b) The heat of the bulb scorches the glass at the top

(c) Dust inside the bulb condenses on the top

(d) Glass undergoes a change due to the heat

Submitted By: Syed Bilawal Shah (Sain)