The Upgrade Accident

**Scenario.** Your tiered submission passes subtask 1 and 2. You add the full-solution tier and now subtask 1 FAILS. Most likely cause? **Which approach is the best fit? The program only needs to print the letter.** **A)** The judge changed **B)** Shared code changed underneath the lower tier — a refactor of parsing or a helper modified behavior the brute-force tier depends on; the upgrade rule (re-run lower-tier tests) was skipped **C)** Brute force cannot fail **D)** The full solution is too slow