If \(U = \mathop e\nolimits^{\frac{{\mathop x\nolimits^2 }}{{\mathop y\nolimits^2 }}} + \mathop e\nolimits^{\frac{{\mathop y\nolimits^2 }}{{\mathop x\nolimits^2 }}} \)then \(x\frac{{\partial u}}{{\partial x}} + y\frac{{\partial u}}{{\partial y}}\)is

• A. u
• B. 0
• C. \(\frac{1}{y}\mathop e\nolimits^{\frac{{\mathop x\nolimits^2 }}{{\mathop y\nolimits^2 }}} + \frac{1}{x}\mathop e\nolimits^{\frac{{\mathop y\nolimits^2 }}{{\mathop x\nolimits^2 }}} \)
• D. \(\frac{{\mathop \partial \nolimits^2 u}}{{\partial x\partial y}}\)