A roller coaster is designed such that riders experience "weightlessness" as they go round the top of a hill whose radius of curvature is 20 m. The speed of the car at the top of the hill should be between

A. 14m/s and 15m/s
B. 15m/s and 16m/s
C. 16m/s and 17m/s
D. 13m/s and 14m/s