Which of the following is true? a. the mean tells us, roughly, how much the data deviate from the average. b. the mean is the more reliable measure of center because it is not affected by extreme values ( outliers)c. the mean of normal distribution is related to the average of the data set. the standard deviation is related to data variation. d. the standard deviation is calculated by adding the data values and dividing the total by the number of data values.