Question: Explain the concept of "income elasticity of demand" and how it can help businesses understand consumer behavior.

A$)$ Income elasticity of demand measures how income levels affect supply and demand in the market.

B$)$ Income elasticity of demand measures the responsiveness of the quantity demanded of a good to changes in consumer income; it helps businesses determine whether a product is a normal good, an inferior good, or a luxury good.

C$)$ Income elasticity of demand measures the impact of government policies on consumer spending.

D$)$ Income elasticity of demand has no relevance to businesses.