The common stock of Air United, Inc., had annual returns of 15.6 percent, 2.4 percent, -11.8 percent, and 32.9 percent over the last four years, respectively. What is the standard deviation of these returns?
A. 13.29 percent
B. 14.14 percent
C. 16.50 percent
D. 17.78 percent
E. 19.05 percent