An object of mass M is attached to a string of negligible mass and spun in a vertical circle of radius R, as shown in the figure above. As the object's height increases, its speed decreases such that the object-Earth system's total mechanical energy remains constant. By how much does the object's kinetic energy decrease in moving from point 0, the lowest point on the circle, to point P, the highest point on the circle?

(A)1/2 MgR
(B)MgR
(C)root2 MgR
(D)2MgR