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3. Suppose that we illustrate demand and supply with quantity on the horizontal axis and income on the vertical axis. Let demand be a function of price and income, (P, I). Which of the following statements is true?
(A) A change in income will cause a shift in the demand curve.
(B) A change in income level is represented by a movement along the demand curve.
(C) Income is treated as an exogenous variable in the graphical analysis.
(D) Price and income together must change in order to create a shift in the demand curve.