Concept:A gas at ordinary pressure does not conduct electricity easily, but when the pressure is reduced, it becomes a conductor.
Explanation:When air is pumped out of a discharge tube, the gas pressure inside falls.
The remaining air molecules become widely separated, allowing free electrons to move without too many collisions.
The high voltage between the electrodes ionises the gas particles, producing ions and free electrons.
These charged particles carry current, so conduction takes place in the tube.
Therefore, the conductivity of the air increases as the pressure is reduced.
Answer:A. the conductivity of the air increases