Among the following statements, what is the most serious problem with the arbitrage pricing theory, and why?

a. Its use of several factors instead of a single market index to explain the risk-return relationship

b. The introduction of nonsystematic risk as a key factor in the risk-return relationship

c. That the APT requires an even larger number of unrealistic assumptions than does the CAPM

d. The model fails to identify the key macroeconomic variables in the risk-return relationship