下列分式中，最简分式是 $\mathit{(}$　　 $\mathit{)}$

A． $\frac{{\mathit{x}}^{\mathit{2}}\mathit{-}\mathit{1}}{{\mathit{x}}^{\mathit{2}}\mathit{+}\mathit{1}}$B． $\frac{\mathit{x}\mathit{+}\mathit{1}}{{\mathit{x}}^{\mathit{2}}\mathit{-}\mathit{1}}$

C． $\frac{{\mathit{x}}^{\mathit{2}}\mathit{-}\mathit{2}\mathit{xy}\mathit{+}{\mathit{y}}^{\mathit{2}}}{{\mathit{x}}^{\mathit{2}}\mathit{-}\mathit{xy}}$D． $\frac{{\mathit{x}}^{\mathit{2}}\mathit{-}\mathit{36}}{\mathit{2}\mathit{x}\mathit{+}\mathit{12}}$