The expected returns for Stocks A, B, C, D, and E are 7%, 10%, 12%, 25%, and 18% respectively. The corresponding standard deviations for these stocks are 12%, 18%, 15%, 23%, and 15% respectively. Based on their coefficients of variation, which of the securities is least risky for an investor? Assume all investors are risk-averse and the investments will be held in isolation.