Which closing is appropriate for a formal complaint letter?
答案解析
**Core Concept**: Writing format (formal correspondence).
**Analysis**: Formal letters require standardized closings.
- **Correct Answer (C)**: 'Yours sincerely' is standard formal closing.
- **Errors**:
- A. Used in semi-formal contexts.
- B/D. Informal/casual expressions.
**Key Point**: Register appropriateness in letter writing.
正确答案：C