OEThe word “during” correctly sets up the meaning, while the participle “making” maintains parallelism.
If you chose (A), remember that “which” typically modifies the immediately preceding noun. This answer choice suggests that Hitler’s campaign was a singular strategic error that ultimately cost him the entire Eastern front, when in fact it was the decision to invade in the winter months that ultimately cost him the entire Eastern front. The use of the coordinating conjunction “and” creates an illogical meaning by assigning equal importance to the phrases “was a singular strategic error” and “that ultimately cost …” The second phrase should be subordinated. When we say something “was A and B”, we are implying that “A” and “B” are of equal importance. In this case, one thing led to another. Hitler made a singular strategic error that ultimately cost him the entire Eastern front. The correct choice should make this clear.
If you chose (B), remember that “which” typically refers to the immediately preceding noun. This answer choice illogically suggests that 1941-42 was a singular strategic error that ultimately cost Hitler the entire Eastern front.
If you chose (C), you correctly removed “which” from the sentence and switched the verb form to the participle “making,” however consider how the word “with” changes the meaning of the sentence. This answer choice incorrectly suggests that Hitler invaded Russia through the 1941-42 invasion of Eastern Europe.
If you chose (E), you correctly removed “which” and “that” from the original sentence. However, it is structurally awkward to have back-to-back modifying clauses before the subject (“Hitler”). Furthermore, this choice adjusts the meaning of the sentence to: “Hitler made a mistake.” Additionally, “made” and “costing” are not parallel, and we now have three modifying clauses instead of two. (D) is more precise.
clauses instead of two. (D) is more precise.
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