A stock has a 50% probability that it will have a return of 10% over the next year, a 25% probability that it will have a return of 20% over the next year, and 25% probability that it will have a return of 20% over the next year. What is the standard deviation of the expected return for this stock over the next year? 15.0% 2.3% 29.7% 22.5%