A thin rod of length l lies on the + x-axis with its left end at the

Question:

A thin rod of length l lies on the + x-axis with its left end at the origin. A string pulls on the rod with a force F directed toward a point P a distance h above the rod. Where along the rod should you attach the string to get the greatest torque about the origin if point P is
(a) Above the right end of the rod?
(b) Above the left end of the rod?
(c) Above the center of the rod?