Let $f(x,y)=2ycos(xy).$ Then the directional derivative of $f$ at the point $(0,1)$ in the direction of a unit vector making an angle $=\frac{}{4}$ with the $x-$axis is

Select one:

$-\sqrt[\phantom{2}]{2}$

$\frac{1}{\sqrt[\phantom{2}]{2}}\frac{1}{2}$

$\sqrt[\phantom{2}]{2}$

$\frac{\sqrt[\phantom{2}]{2}}{2}$

$-\frac{1}{\sqrt[\phantom{2}]{2}}-\frac{1}{2}$