Concept:Thermal expansion can be either useful or harmful depending on the device's purpose.
Explanation:In a fire alarm, thermostat, and bimetallic thermometer, thermal expansion is deliberately used to make the device work.
However, in the balance wheel of a watch, thermal expansion causes the wheel to change size with temperature.
This change alters the moment of inertia and affects the timekeeping accuracy of the watch.
Therefore, thermal expansion is a disadvantage in the balance wheel of a watch.
Answer:D. Balance wheel of a watch