设 $f(x, y)=\left\{\begin{array}{ll}\left(x^2+y^2\right) \cos \left(\frac{1}{\sqrt{x^2+y^2}}\right), & x^2+y^2 \neq 0, \\ 0, & x^2+y^2=0,\end{array}\right.$ 则 $f(x, y)$ 在点 $(0,0)$ 处
A
$\frac{\partial f}{\partial x}, \frac{\partial f}{\partial y}$ 不存在
B
$\frac{\partial f}{\partial x}, \frac{\partial f}{\partial y}$ 连续
C
可微
D
不连续
E
F