Help Detective Miller complete his official, albeit slightly dramatic, incident report by choosing the correct verb form.

By the time I finally breached the suspect's apartment, the heavy-duty paper shredder was dangerously overheated and smoking. It was overwhelmingly obvious that he _____ documents for hours before my arrival.

The correct answer is had been shredding.

We use the Past Perfect Continuous (had been + verb-ing) to show that an action was ongoing up to a specific point in the past. The phrases "for hours before my arrival" and the visible past results (the machine "was dangerously overheated") make this the only grammatically correct choice.

Was shredding is incorrect because of the duration indicated by "for hours before," which requires a perfect tense. Had shredded implies the action was completed as a single event, which doesn't fit the continuous emphasis of "for hours."