Which expression equals \((x-y)(x-y)\) ? Hint: Use the Distributive Property.

\[ \begin{aligned} & x^{2}-y^{2} \\ & x^{2}+y^{2} \\ & x^{2}-2 x y-y^{2} \\ & x^{2}-2 x y+y^{2} \end{aligned} \]