The diagram shows a rectangular loop of current-carrying
wire between the poles of a magnet. The longer sides of the loop are initially
parallel to the magnetic field, and the shorter sides of the loop are initially
perpendicular to the magnetic field. The loop then rotates through so that
all its sides are perpendicular to the magnetic field. Which of the lines on
the graph correctly represents the change in the torque acting on the loop as
the angle its longest sides make with the magnetic field direction varies from
to ?