Find Values Of $x$ And $y$ Such That Both $f_{x}(x, y)=0$ And $f_{y}(x, y)=0$

Multiple Choice

Find values of $x$ and $y$ such that both $f_{x}(x, y) =0$ and $f_{y}(x, y) =0$ .
- $f(x, y) =3200+x y-9 x+8 y$

A) $x=8, y=-9$
B) $x=-8, y=9$
C) $x=-9, y=8$
D) $x=9, y=-8$