Complex Sentence Structure Correction

Very Hard Language And Composition Grammar And Mechanics

Read the following complex sentence:

Although the students prepared extensively for the exam, which they had been studying for weeks, they felt unprepared.

Which option corrects the error in this sentence relating to grammatical structure?

Although the students prepared extensively for the exam, that they had been studying for weeks, they felt unprepared.

Although the students prepared extensively for the exam they had been studying for weeks, they felt unprepared.

Although the students prepared extensively for the exam, which they had prepared for weeks, they felt unprepared.

Although the students prepared extensively for the exam, which they had been studying for weeks, they felt unprepared.

Although the students had extensively prepared for the exam, which they had been studying for weeks, they felt unprepared.

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% Correct53%