Over the past six years, a stock had annual returns of 12 percent, -3 percent, 2 percent, 27 4 percent, 9 percent, and 14 percent, respectively. What is the standard deviation of these returns? Seleccione una: O a. 6.19 percent O b. 5.40 percent O c. 7.08 percent O d. 6.89 percent O e. 6.47 percent