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设 $z=x^y$, 则有
A. $\frac{\partial z}{\partial x}=x^y \ln x$     B. $\frac{\partial z}{\partial x}=y x^{j-1} d x$     C. $\frac{\partial z}{\partial x}=x^y$     D. $\frac{\partial z}{\partial x}=y x^{j-1}$         
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