One of the main problems with the arbitrage pricing theory is $\_\_\_\_\_\_\_\_\_\_.$

Group of answer choices

that the APT requires an even larger number of unrealistic assumptions than does the CAPM

the introduction of nonsystematic risk as a key factor in the risk$-$return relationship

the model fails to identify the key macroeconomic variables in the risk$-$return relationship

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