As the cars of a rollercoaster race down a hill, energy transformations take place. Which of the following is true?

a Kinetic energy is stored in the cars as they move downhill and is released when they move uphill.
b Some of the potential energy is transformed to kinetic energy and other forms of energy.
c Some of the energy is destroyed as the car slows down and comes to a stop.
d The total energy doesn't change; the kinetic energy of the cars is equal to the potential energy of the cars.