The correct answer is D. Diodes, are often utilized as rectifiers, have two electrodes. A diode which means two has two electrodes which are anode and cathode. Electrons are being shot in cathode-anode attracts when the voltage is being applied. Triode means three and it has three electrodes in the vacuum which include grid, anode and the cathode. Grid and cathode offset voltages with a small variation of input. Large variations of the anode will be inflicted by grid voltage within a process called amplification.
