Yuri computes the mean and standard deviation for the sample data set 12, 14, 9, and 21. He finds the mean is 14. His steps for finding the standard deviation are below. s = StartRoot StartFraction (12 minus 14) squared + (14 minus 14) squared + (9 minus 14) squared + (21 minus 14) squared Over 4 EndFraction EndRoot. = StartRoot StartFraction negative 2 squared + 0 squared + negative 5 squared + 7 squared Over 4 EndFraction EndRoot. = StartRoot StartFraction 4 + 0 + 25 + 49 Over 4 EndFraction EndRoot. = StartRoot StartFraction 78 Over 4 EndFraction EndRoot. = StartRoot 19.5 EndRoot. What is the first error he made in computing the standard deviation? Yuri failed to find the difference between each data point and the mean. Yuri divided by n instead of n -1. Yuri did not subtract 9 - 14 correctly. Yuri failed to square -2 correctly.