Bimetallic strips are composed of two different metals bonded together. These strips are designed to bend when subjected to temperature changes due to the difference in the coefficient of thermal expansion between the two metals. Invar, an alloy of iron and nickel, is commonly used in bimetallic strips due to its low coefficient of expansion.

Low coefficient of expansion:
- Invar has a very low coefficient of expansion, meaning it expands and contracts very little with changes in temperature.
- When a bimetallic strip is heated, the metal with the higher coefficient of expansion will expand more than the other metal, causing the strip to bend towards the metal with the lower coefficient of expansion.
- The low coefficient of expansion of invar ensures that the strip will have a predictable and consistent bending behavior.

Other options:
- Low-density: The density of a material does not directly affect its suitability for use in bimetallic strips. The primary consideration is the coefficient of expansion.
- High-temperature resistance: While invar has good high-temperature resistance, this property alone does not make it the ideal choice for bimetallic strips. The low coefficient of expansion is the key factor.
- High abrasion resistance: The resistance to abrasion is not a critical requirement for bimetallic strips, as they are not typically subjected to abrasive conditions.

Conclusion:
Invar is used in bimetallic strips because of its low coefficient of expansion. This property allows the strip to bend predictably when exposed to temperature changes. By ensuring a consistent bending behavior, invar helps bimetallic strips to be used in various applications, such as thermostats, electrical switches, and temperature control devices.