A4. If we simplify the expression $\frac{x-y}{\sqrt{x}-\sqrt{y}}$, we get:
(A) $\sqrt{x}+\sqrt{y}$
(B) $x+y$
(C) $2 x-y$
(D) $x y$
(E) $\sqrt{x}-\sqrt{y}$
A4. The expression in the numerator is written as a difference of squares, which we factorize: $\frac{(\sqrt{x})^{2}-(\sqrt{y})^{2}}{\sqrt{x}-\sqrt{y}}=$ $\frac{(\sqrt{x}-\sqrt{y})(\sqrt{x}+\sqrt{y})}{\sqrt{x}-\sqrt{y}}=\sqrt{x}+\sqrt{y}$.