21 You are trying to decide how to allocate your retirement savings between Treasury bills a ne stock market. The T-Bill rate is 0.12% monthly. You expect the stock market to hav nonthly return of 0.75% with a standard deviation of 4.25%. Let w denote the weight on cock market and 1 w the weight on the T-Bill. (a) Write down a mathematical expression for the portfolio's mean return and standard de ation as a function of w. (b) On a mean-stdev diagram, depict the curve traced out by the portfolio with varying weig