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Which of the following functions can be extended continuously to $\mathbb{R}^{2}$?

$\frac{x^{2}-y^{2}}{x^{2}+y^{2}}$

$\frac{x+y}{x^{2}-y^{2}}$

$\frac{xy}{x^{2}+y^{2}}$

$\frac{x^{3}}{x^{2}+y^{2}}$