[单选题]
A roller coaster loop has a radius $R$ as shown in the diagram. The combined mass of a car and the passengers is $M$.
When the car goes around the top of the loop, the normal reaction force of the track on the car is equal to half of the weight of the car and passengers.
Which expression gives the kinetic energy at the top of the loop?
A. 0
B. $\frac{RMg}{4}$
C. $\frac{3RMg}{4}$
D. $RMg$
