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 _________. Why?