The figure below shows contours of $f(x,y)$, with values of $f$ on the contours omitted.

If $f_{y}(P)$ is positive, answer the following questions about the other derivatives of $f$ at $P$ and $Q$.

(a) What is the sign of $f_x(P)$? ? positive negative zero

(b) What is the sign of $f_x(Q)$? ? positive negative zero

(c) What is the sign of $f_y(Q)$? ? positive negative zero