Joelle consumes food and clothing. For incomes near her current income, her income expansion path is negatively sloped. Indicate whether each of the following statements is true or false, and provide a brief explanation why. (You may want to use a graph in formulating your explanation.)
a. At current income levels, food must be an inferior good. b. At current income levels, clothing must be an inferior good. c. At current income levels, food and clothing must be inferior goods. d. At current income levels, either food or clothing must be an inferior good.