For which values $of$ a and $b$ does the function $f(x)=\frac{ax}{b+x^2}$ have a critical point $atx=2$ and $x=-2?$

$a=1,b=4$

$a=1,b=1$

$a=2,b=2$

$a=4,b=2$

$a=1,b=3$