Concept:Electrical conduction in gases requires ionisation of the gas, and this principle is used in several practical devices.
Explanation:Gas conduction occurs when gas particles are ionised and current flows through the gas.
It is applied in identifying gases because each gas gives a characteristic spectrum when an electric discharge passes through it.
It is also used in lighting and fluorescent tubes, where an ionised gas emits visible light.
Cathode ray oscilloscopes and T.V. tubes also operate using electrical conduction through gases at low pressure.
Photocells do not depend on gas conduction; they work through the photoelectric effect.
So the correct statements are (i), (ii) and (iv).
Answer:B. (i), (ii) and (iv) only