Consider four assets: a, b, c, and d. Their expected returns and standard deviations are given by the table below. a b C d 5% 8% 4% 4% expected return standard deviation 10% 20% 2% 8% The correlation coefficient of assets a and b is 0.4, the correlation coefficient of assets c and d is -1, and the correlation of any other asset pair, i.e., (a, c), (a, d), (b, c), and (b, d), is zero. a) Given the no-arbitrage condition, what is the risk-free rate? (5 points) b) Suppose an investor has the mean-variance utility E(r.) 0.005Ao2, where A = 1. What is the investor's optimal portfolio of assets a, b, c, and d? (10 points)