Concept:Thermal expansion of solids is the tendency of a solid to change its dimensions when its temperature changes.The question asks which device does not rely on this expansion for its operation.Explanation:A simple pendulum keeps time by swinging under gravity, with period T=2πgL.It does not require the expansion of a solid to function; thermal expansion of its rod merely alters the length L and is usually a disadvantage.Fire alarms, thermostats, and bimetallic thermometers all work using a bimetallic strip whose two metals expand at different rates when heated.This differential expansion causes the strip to bend, which is exactly what activates the alarm, controls the switch, or moves the thermometer needle.Hence, devices II, III, and IV are all applications of thermal expansion of solids, but device I is not.Answer:C. I only