大全401 代词

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

答案是d,我想问的是them 指代的是cars, 但是cars是个定语成分而非实意名词，那么them 在这里应该是不可以指代的呀？谢谢