The market risk premium is best approximated by ________. (Please explain why which answer picked is correct)

A.) the difference between the return on an index fund and the return on Treasury bills

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

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

D.) the difference between the return on the highest-yielding asset and the return on the lowest-yielding asset