Determine the sign of $f_x$ and $f_y$ at each indicated point using the contour diagram of $f$ shown below. (The point $P$ is that in the first quadrant, at a positive $x$ and $y$ value; $Q$ through $T$ are located clockwise from $P$, so that $Q$ is at a positive $x$ value and negative $y$, etc.)

(a) At point $Q$,
$f_x$ is ? positive negative and
$f_y$ is ? positive negative .

(b) At point $R$,
$f_x$ is ? positive negative and
$f_y$ is ? positive negative .

(c) At point $S$,
$f_x$ is ? positive negative and
$f_y$ is ? positive negative .