Consider a treasury bill with a rate of return of 5% and the following risky securities: security a: e( r) = .15; variance = .0400 security b: e( r) = .10; variance = .0225 security c: e( r) = .12; variance = .1000 security d: e( r) = .13; variance = .0625 the investor must develop a complete portfolio by combining the risk-free asset with one of the securities mentioned above. the security the investor should choose as part of her complete portfolio to achieve the best cal would be