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

Sigma Subscript D = 1.05; this pond can expect the difference of frogs and koi to vary by approximately 1.05 from the mean.
Sigma Subscript D = 1.1; this pond can expect the difference of frogs and koi to vary by approximately 1.1 from the mean.
Sigma Subscript D = 3.1; this pond can expect the difference of frogs and koi to vary by approximately 3.1 from the mean.
Sigma Subscript D = 13; this pond can expect the difference of frogs and koi to vary by approximately 1.05 from the mean.