Let $f(x, y) = \begin{cases} \frac{x^2 - y^2}{x + y} & \text{if } (x, y) \neq (0, 0) \\ 4 & \text{if } (x, y) = (0, 0) \end{cases}$ \\ Then $f(0, 0) = $ \\ and $\lim_{(x, y) \to (0, 0)} f(x, y) = $ \\ So $f(x, y)$ Select an answer continuous at $(0, 0)$.