Which of the following pairs describes two terms used to describe stiffness in a material?

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

Answer: (B)

Stiffness is about how strongly a material resists deformation and how it responds when pressure is removed. Elasticity describes the ability of a material to return to its original shape after a deforming force is removed, which directly relates to resisting permanent change. Compressibility measures how much a material’s volume changes under pressure, and stiff materials tend to have low compressibility. Put together, elasticity and compressibility cover both the tendency to recover shape and the resistance to compression, which together describe stiffness.

Other pairings don’t fit as well because density tells you how much mass is in a volume, not how deformation behaves under load; viscosity describes flow resistance in fluids, not how a solid resists being deformed; combining these with elasticity or compressibility doesn’t center on stiffness in the same way.