**Core Concept:** Past perfect tense indicates an action completed before another past action. **Answer Analysis:** - **B** is correct. 'Had finished' (past perfect) shows the homework was completed before 'the movie started' (simple past). - **A** incorrectly mixes present perfect ('has left') with simple past ('arrived'). - **C** uses past continuous ('were eating') with present tense ('rings'), creating inconsistency. - **D** uses present continuous ('am reading') with a past time reference ('last week'), which is illogical. **Common Mistakes:** Confusing past perfect with present perfect or simple past tense.