The degree of stiffness may also be described by these 2 terms:

• A. Elasticity and Compressibility
• B. Elasticity and Density
• C. Density and Viscosity
• D. Compressibility and Viscosity

Answer: (A)

Stiffness is about how much a material resists changing shape or volume when a force is applied. Two ways we describe that resistance are elasticity and compressibility.

Elasticity tells us how the material responds to stress in terms of shape. A stiffer material deforms less under the same load, and if it’s elastic, it returns to its original shape when the load is removed. The higher the elastic modulus, the stiffer the material is in resisting shape change.

Compressibility describes how the material’s volume changes under pressure. A material that is less compressible resists squeezing and doesn’t shrink much, which is another aspect of being stiff. Low compressibility means the material holds its volume up under pressure.

Together, these two terms cover how a material resists deformation in two different ways: bending or stretching (elasticity) and changing size (compressibility). Density and viscosity aren’t direct measures of stiffness—they relate to mass per volume and flow resistance, respectively, rather than how much deformation occurs under load.