Concept:Gas conduction at low pressure occurs through electron emission from an unheated cathode when a strong electric field is applied.
Explanation:At low pressure, gases become conducting when a very high voltage is applied across the tube.
The intense electric field draws electrons out from the cold cathode.
This form of discharge is known as cold cathode emission.
Because the emission is caused by the strong electric field, it is also described as field emission.
Hot cathode emission and thermo ionic emission both require a heated cathode, which is not essential in low-pressure gas conduction.
Hence, the correct terms are III and IV.
Answer:C. III & IV