Here are the corrections for each sentence:

21. **B** - "to see" should be "seeing." The correct phrase is "looking forward to seeing you again."

22. **A** - The phrase "that was published" should be "which was published." The correct usage is "Her novel, which was published last month..."

23. **B** - "interested" should be "interesting." The correct form is "an interesting holiday."

24. **A** - "complain" should be "complained." The correct form is "Many people have complained."

25. **C** - "was she going" should be "she was going." The correct structure is "where she was going."