When two actions happen in the past, it is necessary to show which action happened earlier than the other. The earlier action is expressed in the Past Perfect tense and the later action in Simple Past. Look at the examples given below: They had reached the school before the bell rang. ↓ ↓ (Earlier Action) (Later Action) The patient died after the doctor had come. ↓ ↓ (later action) (earlier action) In the given question: All the students had left (Earlier Action - Past Perfect) before the lecturer arrived (Later Action - Simple Past). This is the Correct Sentence. Thus, the error is in Part 3 of the given sentence, we will omit 'had' to make the sentence correct.