**Core Concept**: Grammar (third conditional). **Analysis**: - The sentence describes a past hypothetical situation. The correct structure is 'had + past participle' in the if-clause. - Option C ('had left') fulfills this requirement. - Option A/B use present/simple past tense, violating third conditional rules. Option D ('has left') introduces present perfect, which is incompatible. **Key Point**: Third conditional requires past perfect in the condition clause. **Error Alert**: Mistaking third conditional for second conditional (e.g., choosing 'left').