Q: An Income Elasticity of Demand eᵢ is (a) (Q₂ − Q₁)/(I₂ − I₁) × (I₂ + I₁)/(Q₂ + Q₁) (b) (Q₂ + Q₁)/(I₂ − I₁) × (I₂ + I₁)/(Q₂ + Q₁) (c) (Q₂ − Q₁)/(I₂ + I₁) × (I₂ − I₁)/(Q₂ + Q₁) (d) (Q₂ − Q₁)/(I₂ + I₁) × (I₂ + I₁)/(Q₂ + Q₁) where: Q₁ is quantity demanded in the base year Q₂ is quantity demanded in the following year I₁ is income level in the base year I₂ is income level in the following year