Veritas Prep Official explanation

Correct Answer: B

This sentence uses the "not only x but also y" construction, which requires that x and y be grammatically parallel. The first term, "on the...training" is a prepositional phrase, and so the underlined portion must also begin with a preposition. A (beginning with a pronoun), C (beginning with a conjunction), and E (beginning with a participle) can therefore be eliminated. Answer D's "and they enabled them" incorrectly elevates the importance of the end of the sentence by restating it as a new independent clause. It also uses the pronouns "they" and "them" to refer to two different antecedents, and can be eliminated for this ambiguous pronoun reference error.. In contrast, the correct answer B uses "who were then able" to turn the end of the sentence into a relative clause, which correctly modifies the "armies" referenced immediately before.
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Choice A, C & E having parallelism error as highlighted.
Choice D is having meaning error.

IMO B
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The expansion of the Mongol empire depended not only on the Mongols' rigorous training and the discipline of their soldiers but also they implemented a highly organized supply and communication network among their many widespread armies and then enabling them to receive their current orders and some replenishment of their weaponry.


A. they implemented a highly organized supply and communication network among their many widespread armies and then enabling them Incorrect

B. on their implementation of a highly organized supply and communication network among their many widespread armies, who were then able Correct

C. because of their implementation of a highly organized supply and communication network among their many widespread armies, who they enabled Incorrect

D. on their implementing of a highly organized supply and communication network among their many widespread armies and they enabled them Incorrect

E. implementing a highly organized supply and communication network among their many widespread armies, enabling them Incorrect

correct parallelism - ................not only on .............................but also on.................
eliminate A, C & E

D - implementing uses wrong, they introduce parallelism error and also they refers to what?

Answer - B