**Core concept**: Grammar (future perfect tense). **Analysis**: The phrase 'By the time he arrives' indicates a future action completed before another future event. The correct tense is **A. will have left**. **Wrong options**: - B. 'left' uses simple past tense for a completed future action (illogical). - C. 'leaves' mismatches subject-verb agreement (plural 'we' vs. singular 'leaves'). - D. 'is leaving' uses present continuous for a future action but lacks completion emphasis. **Key point**: Future perfect tense (will have + past participle) expresses completion before a future time.