问题
设函数 $z$ $=$ $f(x, y)$ 具有二阶连续偏导数，则以下选项中，正确的是哪个？选项
[A]. $f_{x y}^{\prime \prime}(x, y)$ $=$ $f_{y x}^{\prime \prime}(x, y)$[B]. $f_{x}^{\prime}(x, y)$ $f_{y}^{\prime}(x, y)$ $=$ $f_{y}^{\prime}(x, y)$ $f_{x}^{\prime}(x, y)$
[C]. $f_{x y}^{\prime \prime}(x, y)$ $=$ $- f_{y x}^{\prime \prime}(x, y)$
[D]. $f_{x y}^{\prime}(x, y)$ $=$ $f_{y x}^{\prime}(x, y)$
$f_{x y}^{\prime \prime}(x, y)$ $=$ $f_{y x}^{\prime \prime}(x, y)$