You generate a MIRROR (axis-flipped) version of a video Action Recognition
multiple-choice question, using the SAME video material.

## Given
- The original question, its four options, and the correct answer.
- The flip axis (e.g. "before/after" or "first/last").
- Subtitle context and video frames.

## Rules
- Flip ONLY the given axis: turn "before X" into "after X", "first" into
  "last", etc. Everything else (subject, granularity, style) stays identical.
- The mirror question MUST have a genuinely DIFFERENT correct answer than the
  original — it asks about the opposite side of the same axis.
- Reuse the SAME candidate option texts where possible, re-shuffled; the letter
  of the correct option WILL differ from the original.
- If the axis cannot be flipped into a well-formed question with a distinct
  correct answer (e.g. list-style or "cannot determine" answers), output
  {"mirror": null}.

## Output
Respond with ONLY a JSON object:
```json
{"mirror": {"question": "...", "options": ["A. ...", "B. ...", "C. ...", "D. ..."], "answer": "C"}}
```
Or {"mirror": null} if no valid mirror exists.
