已知函数 $f(u)$ 可导且 $f^{\prime}(0)=2$, 设 $z=f\left(\arctan \frac{x}{y}\right)$, 则 $z=\left.\frac{\partial z}{\partial x}\right|_{(0,2)}$ 与 $z=\left.\frac{\partial z}{\partial y}\right|_{(0,2)}$ 的值依次为
A
$\frac{1}{2}, \frac{1}{2}$.
B
$\frac{1}{2},-\frac{1}{2}$.
C
1,0.
D
2,0 .
E
$1,-1$.
F