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设 $f^{\prime}\left(x_0\right)$ 存在, 则 $\lim _{\Delta x \rightarrow 0} \frac{f\left(x_0-\Delta x\right)-f\left(x_0\right)}{\Delta x}=$.
A. $f^{\prime}\left(x_0\right)$     B. $-f^{\prime}\left(x_0\right)$     C. $2 f^{\prime}\left(x_0\right)$     D. 不存在         
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