Why can the sum $x^2+y^2$ not be factored into binomials with real coefficients using the difference-of-squares rule?

Because the difference‑of‑squares rule applies to $a^2-b^2$, not $a^2+b^2$

Because $x$ and $y$ must be equal for factoring to work

Because one of the terms must be a negative square

Because $x^2+y^2$ factors only when $x$ and $y$ are constants