The Olympic Games helped to keep peace among the pugnacious states of the Greek world in that a sacred truce was proclaimed during the festival’s month.
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The Olympic Games helped to maintain peace among the Greek states. How? A truce was declared during the month the games were held.
“In that” means “in a way that”. In this context, this use looks appropriate since the games helped maintain peace in a way that a truce was declared during the time of the games. However, per OG explanation, “in that” is overly formal and stilted, and therefore, ‘for’ should be preferred over the same.
Also, “festival’s month” is an informal way of saying the idea “the month of the festival”. On GMAT, we should prefer the latter way.
(A) Incorrect. For the problems mentioned above.
(B) Incorrect. “proclaiming” is a verb-ing modifier following a clause and preceded by a comma. Therefore, it acts as a clause modifier. However, it doesn’t make sense with the subject of the preceding clause. The Games cannot proclaim anything!
Also, “month of the festival” is preferred over “festival’s month”.
(C) Incorrect. There is no antecedent of “they”. “they” cannot logically refer to ‘Olympic Games’. Also, “month of the festival” is preferred over “festival month”.
(D) Correct. “in that” has been replaced by “comma+for”, which introduces an independent clause. “for” in this construction means ‘because’. The sentence is logical and has no grammatical errors.
(E) Incorrect. Like option B, this option also indicates that the Olympic Games did the proclamation! Also, this option distorts the meaning of the sentence by presenting the time of the truce in the form of a relative clause. The original sentence meant that the truce was proclaimed during the month of the festival. This option, on the other hand, means that the sacred truce was always for the month of the festival (since this information is presented using a ‘that’ clause modifying ‘sacred truce’).