Please show all work and explain reasoning. Thank you !!

8) Consider a graph on which one good Y is on the vertical axis and the only other good X is on the horizontal axis. On this graph the income-consumption curve has a positive slope for low incomes, then it takes a zero slope for a higher income, and then it takes a negative slope for even higher incomes (the curve looks like an arc, first rising and then falling as income increases). This curve illustrates that A) both X and Y are normal for all income levels. B) only Y is normal for all income levels. C) only X is normal for all income levels. D) both X and Y are inferior for all income levels