Alex’s times for running a mile are Normally distributed with a mean time of 5.28 minutes and a standard deviation of 0.38 seconds. Chris’s times for running a mile are Normally distributed with a mean time of 5.45 seconds and a standard deviation of 0.2 seconds. Ten of Alex’s times and 15 of Chris’s times are randomly selected. Let represent the difference in the mean times for Alex and Chris. Which of the following represents the standard deviation of the sampling distribution for ?