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