Concept:A transistor works as both a switch and an amplifier, but it does not replace a thermostat in electric irons.
Explanation:Statement I is correct: In digital circuits, a transistor acts as a fast electronic switch. It is either fully ON (saturation mode) or fully OFF (cutoff mode).
Statement II is correct: In analog circuits, a small alternating signal at the base controls a larger current, so the transistor amplifies weak alternating signals.
Statement III is incorrect: Electric irons prevent overheating using a bimetallic strip thermostat, not a transistor. The thermostat bends when heated and breaks the circuit.
Therefore, the valid uses are I and II only.
Answer:B. I and II only