[单项选择]An analyst does research about calculating the expected standard deviation of returns. An investor estimates that the retum from a security over his holding period will be either 12 percent or 16 percent and that each of the two potential returns has an equal probability of occurrence. The expected standard deviation of returns for that security over the investor's holding period is closest to:() A. 0.020 B. 0.028 C. 0.040