A Small Car of Mass M Travels Along a Straight

Multiple Choice

A small car of mass M travels along a straight, horizontal track. As suggested in the figure, the track then bends into a vertical circle of radius R.
-What is the minimum acceleration that the car must have at the top of the track if it is to remain in contact with the track?

A) 4.9 m/s², downward
B) 4.9 m/s², upward
C) 9.8 m/s², downward
D) 9.8 m/s², upward
E) 19.6 m/s², upward