设 f(x,y) = } (x^2 + y^2) arctan (1)/(x^2 + y^2), & (x,y) neq (0,0) 0, & (x,y) = (0,0) 则 f_y(0,0) = ( )A. 1B. 0C. 不存在D. pi/2

设 $f(x,y) = \begin{cases} (x^2 + y^2) \arctan \frac{1}{x^2 + y^2}, & (x,y) \neq (0,0) \\ 0, & (x,y) = (0,0) \end{cases}$ 则 $f_y(0,0) = (\quad)$

A. 1

B. 0

C. 不存在

D. $\pi/2$