A Roller Coaster Starts from Rest at a Height H

Multiple Choice

A roller coaster starts from rest at a height h at the left side of a loop-the-loop,as shown in the figure.It is not attached to the track in anyway,and there is no friction from the track or from air resistance.If the radius of the loop is R = 6.0 m,what is the minimum height h for which the roller coaster will not fall off the track at the top of the loop?

A) 21 m
B) 18 m
C) 15 m
D) 12 m
E) 8.5 m