Difficulty: Hard
Correct Answer: bacab
Explanation:
Introduction / Context: Another anchor-based completion: substrings like “bbca”, “bcca”, and “ac…cb” must be preserved. We insert five letters to keep those anchors intact and create smooth joins.
Given Data / Assumptions:
Concept / Approach: Favor fills that maintain “bbca” then “bcca”, and finally produce a clean “ac…a…cb” ending, avoiding triple-letter collisions.
Step-by-Step Solution: Insert "bacab": b bbca a bcca b ac a a cb. The anchors are preserved in order, and joins remain consistent without forced breaks.
Verification / Alternative check: Competing fills either break “bcca” or leave an untidy tail before “cb”.
Why Other Options Are Wrong: abeba / acbab / bcaajb: Each damages at least one anchor or creates awkward double a/b clusters at boundaries.
Common Pitfalls: Not checking the last two joins; the final “…a _ cb” part is especially sensitive.
Final Answer: bacab
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