Identify the correct statements from the following:

A diode valve consists of two electrodes (cathode and anode). It allows unidirectional flow of electrons (from cathode to anode) and thus can operate as a rectifier. It does not have a control grid and therefore cannot amplify a signal.

A triode valve consists of three electrodes (cathode, control grid, and anode). Like a diode, it allows unidirectional flow of electrons, meaning it can also operate as a rectifier. Furthermore, the control grid allows a small input voltage to control a large output current, enabling the triode to operate as an amplifier.

Thus, statements (2), (3), and (4) are correct.