AMC 10 2008b Problem 5

For real numbers $a$ and $b$, define $a * b=(a-b)^2$. What is $(x-y)^2*(y-x)^2$? $\mathrm{(A)}\ 0 \mathrm{(B)}\ x^2+y^2 \mathrm{(C)}\ 2x^2 \mathrm{(D)}\ 2y^2 \mathrm{(E)}\ 4xy$