There are frogs and koi in a pond and the number of frogs and the number of koi in the pond are independent. Let the x represent the number of frogs in any given week, and let y represent the number of koi in any given week. X had a mean of 28 with a standard deviation of 2.7, and y has a mean of 15 with standard deviation of 1.6. Which answer choice correctly calculates and interprets the standard deviation of the difference, D=X-Y?