The market risk premium is best approximated by the

Multiple Choice

difference between the return on an index fund and the return on Treasury bills

difference between the return on a small$-$firm mutual fund and the return on the Standard & Poor's $500$ Index

difference between the return on the risky asset with the lowest returns and the return on Treasury bills

difference between the return on the highest$-$yielding asset and the return on the lowest$-$yielding asset