A roller coaster is designed such that riders experience "weightlesness" 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 is between: (a) 14 m/s and 15 m/s (c) 16 m/s and 17 m/s (b) 15 m/s and 16 m/s (d) 13 m/s and 14 m/s