19. The "Efficient Frontier", also known as the "Investment Opportunity Set", is a graph of a. the relationship between the yield to maturity and the time to maturity for risk free treasury securities b. the relations hip between the dividend yield of a stock and the required rate of return on the stock c. the probability of having a return that is plus or minus one standard deviation from the expected return d. the relationship between expected return and standard deviation for portfolios made up of various combinations of risky assets