A \( \left(x^{2}+y^{2}\right)^{2}=a \) \( \left(x^2-y^2\right) \)
B \( \left(x^{2}-y^{2}\right)^{2}=a\left(x^{2}+y^{2}\right) \)
C \( \left(x^{2}+y^{2}\right)^{2}=a^{2}\left(x^{2}-y^{2}\right) \)
D \( \left(x^{2}-y^{2}\right)^{2}=a^{2}\left(x^{2}+y^{2}\right) \)