**Core Concept:** Grammar - Past perfect tense **Answer Key:** C **Analysis:** - **C** correctly uses past perfect ('had operated') to show an action completed before another past event ('error occurred'). - **A** uses simple past for sequential actions without time relation. - **B** uses present perfect for ongoing actions. - **D** uses past continuous for simultaneous actions. **Common Mistake:** Misusing simple past (A/D) when describing sequential past events.