Concept:For isotropic materials, cubic expansivity is three times linear expansivity.Explanation:Linear expansivity, α, measures the fractional change in one dimension per degree change in temperature.Cubic expansivity, γ, measures the fractional change in volume per degree change in temperature.Volume expands in three independent directions, so the cubic expansivity is the sum of the linear expansivities in all three directions.Thus, γ=α+α+α=3α.Answer:D. γ=3α