Correct Response: B. Standard deviation is a measure of the variability of a data set. It is the square root of the variance of the data set, where the variance represents how spread out a data set is from the mean. In this way standard deviation is useful for determining the variability of a river's discharge over a period of time. The median (A) measures the middle value of a data set not its variability, while the mode (C) is the most common value in the data set, and the arithmetic mean (D) is the average of the data set, a value that the hydrologists have already calculated and that does not measure variability.
