The key to control over the Eurasian steppes lay in the nomad’s abilit

Hi Experts,
The subject of the sentence is "The Key".. Verb is "Lay"

Isnt this a singular subject and plural verb? Shudnt this be "The Key.... lays"..... (Though this is in the non-underlined part...)

Here's the official explanation provided by the GMAC for this question:

In this sentence the word both introduces the first of two conjoined phrases describing ways that nomads were able to use horses. These phrases must be presented in parallel form, in this case starting with the preposition as . Idiomatically, when both introduces the first of two conjoined phrases, and must introduce the second.

Option A: Since both introduces the first of two conjoined phrases, it is unidiomatic for but also to introduce the second.

Option B: Since both introduces the first of two conjoined phrases, it is unidiomatic for or to introduce the second.

Option C: Correct. B oth introduces as a means of transport, the first of two conjoined phrases. And introduces the second of these phrases, as an effective military tool. The two phrases are presented in parallel form, both starting with as.

Option D: When both introduces the first of two conjoined phrases, it is unidiomatic for or to introduce the second. Furthermore, as a means of transport is not parallel with an effective military tool; the former phrase starts with the preposition as, while the latter starts with the article an and completes the conjunction illogically with a noun phrase rather than an adverbial phrase.

Option E: As a means of transport is not parallel with also an effective military tool; the former phrase starts with the preposition as, while the latter starts with the adverb also.

The correct answer is C.

Please note that I'm not the author of this explanation. I'm just posting it here since I believe it can help the community.

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