In terms of physics, the characteristic feature of the roller coaster

Hi Manoj,
are u sure the OA is 'C' ?? Do you know the source of the question ? I had zeroed in on C, but then after thinking some more (which sometimes screws the answer:)) opted for D.

If we look at the last section of the sentence ..'has been converted to kinetic energy by the time the ride ends' and the first part of option D -' potential energy of the cars', it make sense.. i.e. 'potential energy of the cars has been converted to kinetic energy by the time the ride ends' . However with 'C', we read it as 'potential energy from the cars 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'... this is awkard and makes the sentence look lengthy(even though it isn't)....

These are the questions which send the scores for a toss.. I hate such questions...

(A) cars' potential energy, gained through their being lifted by a chain drive
- No clear referent for “their”. Hence A is out.
(B) cars' potential energy, a gain achieved as they are lifted by a chain drive
- No referent for “they”. Hence B is out.
(C) potential energy from the cars' being lifted by a chain drive
- The perfect tense indicates that all actions are completed; “being” suggests as if the actions are going on. Hence C is out.
(D) potential energy of the cars, gained as a chain drive lifts them.
- Removes all the errors and hence the correct Choice
(E) potential energy gained by the cars, being achieved while a chain drive lifts them
-The perfect tense indicates that all actions are completed; “being” suggests as if the actions are going on. Hence E is out.

(A) cars’ potential energy, gained through their being lifted by a chain drive (their -- ??, doesnt looks right)
(B) cars’ potential energy, a gain achieved as they are lifted by a chain drive (a gain is the characteristic of roller coaster?? - meaning issue)
(C) potential energy from the cars’ being lifted by a chain drive (being is generally avoided)
(D) potential energy of the cars, gained as a chain drive lifts them -- this seems okay
(E) potential energy gained by the cars, being achieved while a chain drive lifts them (same as C)

A tricky one indeed

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 - Incorrect: Reason highlighted, ',' Comma is not required after energy.
(B) cars' potential energy, a gain achieved as they are lifted by a chain drive . - Incorrect : Pronoun they refers that cars' potential energy is lifted.
(C) potential energy from the cars' being lifted by a chain drive. - Incorrect "energy from" is wrong; moreover the statement says cars generate potential enery and at the same time they are being lifted.
(D) potential energy of the cars, gained as a chain drive lifts them. - Correct - Proper Idiom. Proper Tense
(E) potential energy gained by the cars, being achieved while a chain drive lifts them. Incorrect

Kudos will be appreciated.

in answer D "...cars, gained..."gained modify cars or is separated from previous clause ?
could someone explain structure ? which is related to "gained" modify ?

So this is where modifiers get a little complicated. We ofter think of most modifiers as describing the closest noun, but they can also often modify an entire noun phrase. (i.e. noun + prepositional phrase). In other words, the modifier can describe either the noun from the prepositional phrase to OR the noun the prepositional phrase describes.

In D, our modifier ("gained ... drop") follows the noun phrase "the potential energy of the cars". Here, the modifier is immediately next to the noun "cars", so the modifier could describe "cars" ... but it could also describe then entire noun phrase, so it could also describe "potential energy".

Thinking about this logically, does it make more sense for cars to be gained by lifting roller coast cars up to the first drop or for potential energy to be gained by lifting roller coaster cars up to the first drop? In the first case, roller coasters would need to gain cars as they move, which makes zero sense. However, it makes a lot of sense for roller coasters to gain potential energy as they move. So while the modifier grammatically could describe either "cars" or "potential energy", it must describe "potential energy" based on meaning.

Takeaways:
1) Watch out for noun phrases when dealing with modifiers
2) Check for logical meaning

In option D "Them" refers to roller coaster or cars ? if its for car then it incorrectly means as if potential energy of car is lifted by a chain.
please explain

"Them" is a plural pronoun, so it must refer to a plural noun. "Potential energy" and "roller coaster" are both singular, so they cannot be the subject of "them". So "them" does refer to "cars" (the only plural noun), but this is where your interpretation is flawed — this means that the "cars" (NOT the "potential energy of the cars") are lifted to the top of the first drop by a chain drive. This process creates potential energy.

Here, the modifier "gained" refers to the whole noun phrase "potential energy (of the cars)", while the pronoun "them" within the modifier refers to just the noun from the prepositional phrase "cars". This creates a sentence that is both logical and grammatical.

Look out for pronoun agreement — it can tell you a lot. And again, be sure to look out for nouns vs. noun phrases. They can both be subjects for modifiers/pronouns, but you need to be sure which one you're using.

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