Please underline the portion of the sentence to be changed.

The verb needs to agree with substances (not one) and be in present simple tense (because we are talking about a persistent, general truth). This leaves answer choices A and B. B is worded awkwardly, using at which time instead of simply saying when. Also, B uses both... or..., though it should be both... and....

Therefore, I would choose answer choice A.
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My GMAT debrief

The very easy sentence challenges us the usage of parallel idiom BOTH x AND y. Only choice A matches this rule. We just scan through all at But, and, or words.
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@jainvik7



I am afraid one cannot justify such a wide remark. You may refer to Stacey’s observation in the link you have cited.


You may thus see it is not always that 'one of the construct' is singular, as you have mentioned. Secondly, in this present example, you see the prevalence of the subordinate clause introduced by the noun modifier ‘that’ . So the verb is going to be plural in this case, namely ‘tell’ rather than the singular ‘tells’.

Am I right?
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Consider these examples.

One of the books in the rack has been written by Irving Wallace

This is a simple sentence with ‘one ‘as the subject, of the books as the prepositional intermediary and ‘is ‘as the verb. In this case, the subject and the verb are singular.

Look at the next case. One of the books that have been written by Irving Wallace is in the rack. --- here the main clause is ‘one of the books is in the rack’ and the subordinate clause introduced by the relative pronoun and the conjunction is ‘that have been written by Irving Wallace’. The verb of this subordinate cluse will be plural, since the relative pronoun refers to the noun before and takes on its characteristics such as number and gender. Hence, the subject of the subordinate clause is plural in nature and the verb is plural.

This is what Stacey says according to me. In other words, whenever you see a relative pronoun such as that or who etc, in a complex sentence that contains ‘ one of the construct’ , be guided by the number of the noun it touches for determining the number of the verb.
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Alicia Helle, an online student at the UW, "Obtaining my degree online has been a blessing. With two small children, I am able to work when it is convenient for my family and me. I have nothing but positive comments and experiences from my time at UW-Stout.”

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Biologists believe that they have found one of the substances that tell individual genes both when to become active and when to remain quiescent in the earliest phases of an embryo’s development.

clause initiators which-nonrestrictive- and that - restrictive - depends on the context of sentence. THAT IS used as a function word immediately before or after a word group consisting of a verbal auxiliary or a form of the verb be preceded by there or a personal pronoun subject to indicate emphatic repetition of the idea expressed by a previous verb or predicate noun or predicate adjective

usully the formula is to take the one after eliminating the "of" construction...but "it" depend on meaning of the sentence
take this one.

In the mid-1920s the Hawthorne Works of the Western Electric Company was the scene of an intensive series of experiments that would investigate changes in working conditions as to their effects on workers' performance.

(A) that would investigate changes in working conditions as to their effects on workers' performance
(B) investigating the effects that changes in working conditions would have on workers' performance
(C) for investigating what the effects on workers' performance are that changes in working conditions would cause
(D) that investigated changes in working conditions' effects on workers' performance
(E) to investigate what the effects changes in working conditions would have on workers' performance

FIRST of all you must think who plays strong role on subsequent verb following " THAT" in this example just read as
AN intensive investigating changes in woring conditions or effects change in working conditions. which one is better? definitely first one is better


same principle goes to you concern that: one tells individual genes bo............ OR substances tell individual genes bo..............SURELY the second one is better
have got the point i wanted to share ?

THAT in the original sentence modifies the antecedent ONE, not the SUBSTANCES, so we can eliminate choice (A), (B) and (E). Between choice (C) and (D), choice (D) is better because choice (C) makes an ambiguous meaning whether the verb REMAIN is paralleled with ACTIVE or BECOME. In this case, we can easily discover that REMAIN will parallel with BECOME, not ACTIVE. So, we need the parallel term WHEN TO.
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