In terms of physics, the characteristic feature of the roller coaster is that the cars' potential energy, gained through their being lifted by a chain drive through the Earth's gravity to the top of the first drop, has been converted to kinetic energy by the time the ride ends.
(A) cars' potential energy, gained through their being lifted by a chain drive
(B) cars' potential energy, a gain achieved as they are lifted by a chain drive
(C) potential energy from the cars' being lifted by a chain drive
(D) potential energy of the cars, gained as a chain drive lifts them(D)
(E) potential energy gained by the cars, being achieved while a chain drive lifts them
D答案中them 读着好像指代potential energy,不指代cars,觉得指代错误.请nn指点.谢谢了! | 
