请教一道语法题

1.        Framed by traitorous colleagues, Alfred Dreyfus was imprisoned for twelve years before there was exoneration and his freedom.

401  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

哪位大侠能给偶讲一下为什么第二题中的A不对, 我觉得和第一题一样都应该强调被动. 是gained through的错么?

cars' potential energy is unidiomatic;

gained through their being lifted: unnecessary passive mood. as also is wordy and awkward;

"gained through their being lifted: unnecessary passive mood. as also is wordy and awkward;"

那第一题中的being exonerted and freed 就不awkward?

"cars' potential energy is unidiomatic" 为什么? my potential energy 错么? 一定要说energy of mine?