Explanation: A vacuum tube's classification is based on the number of active electrodes it contains. A triode, as indicated by the prefix "tri-" (meaning three), has three primary elements: 1. **Filament or Cathode:** This element is heated to emit electrons. The filament can act as both the heater and the electron emitter, or a separate cathode sleeve can be heated by an internal filament. 2. **Control Grid:** Placed between the cathode and the plate, this wire mesh or helix is used to control the flow of electrons from the cathode to the plate, thus amplifying or switching the signal. 3. **Plate (Anode):** This is the positive electrode that attracts the electrons emitted from the cathode, completing the circuit. A diode (two elements) lacks a control grid, while a tetrode (four elements) adds a screen grid, and a pentode (five elements) further adds a suppressor grid. Therefore, a tube with only a cathode, grid, and plate is definitively a triode.