You decided to purchase Renfrow Corp. stock five years ago. The stock has had an annual returns of -7 percent, 9 percent, 8 percent, -2 percent, and 5 percent for the past five years, respectively. What is the standard deviation of returns for this stock? a. 7.84% b. 6.26% c. 6.88% d. 7.17% e. 5.61%