Concept:Electrons are released from the cathode by thermionic emission.
Explanation:Inside a cathode-ray tube, a metal filament acts as the cathode.
When the filament is heated, it gains enough thermal energy to release electrons.
This release of electrons due to heat is called thermionic emission.
These free electrons are then accelerated towards the anode and controlled by electric or magnetic fields.
Thus, the source of the electrons is the heated metal filament, not ionization or the plates.
Answer:A. heating a metal filament.