Which sentence uses proper subjunctive mood?
答案解析
**Core Concept**: Grammar (subjunctive mood).
**Analysis**: Subjunctive requires base verb form after demands/suggestions.
- **Correct Answer (C)**: 'Improve' (base form) follows 'demand that'.
- **Errors**:
- A/B. Use conjugated verbs ('checks', 'operates').
- D. Uses progressive tense instead of base verb.
**Key Point**: Subjunctive structure 'that + subject + base verb'.
正确答案：C