Astudent calculates the potential energy of a roller coaster at the top of a hill. then, she uses the kinetic energy equation to calculate the roller coaster's energy as it reaches the bottom of the hill. the calculations show that the roller coaster's potential energy was slightly greater at the top of the hill than its kinetic energy at the bottom of the hill. which of the following best explains this difference? a. energy in moving systems is not always conserved. b. the roller coaster's velocity decreases as it moves downhill. c. the student did not account for energy transformed into heat and sound. d. the gravitational force is greater at the top of a hill than at the bottom.