The average of x numbers is $y^{2}$ and the average of y numbers is $x^{2}$. So the average of all the numbers taken together is:
[A] $xy$
[B] $xy^{2}+yx^{2}$
[C] $\frac{x^{2}+y^{2}}{x+y}$
[D] $\frac{x^{3}+y^{3}}{x+y}$