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Complete the cascade “ab → bcbc → caca” by choosing the 4-letter fill that preserves the stepping from a→b→c cycles: a _ ab_bcbc_caca_

Difficulty: Medium

ccba
acba
bccb
bcab
None of these

Correct Answer: bcab

Explanation:

Introduction / Context:
The visible sub-blocks “ab”, “bcbc”, and “caca” indicate a staircase: each block starts with the next letter in a→b→c and alternates with its successor (ab, bc bc, ca ca). The correct 4-letter fill must connect these blocks smoothly from the initial “a”.

Given Data / Assumptions:

Scaffold: a _ ab _ bcbc _ caca _
Four letters are inserted sequentially.
We must preserve the stepping first letters a→b→c and the internal alternations.

Concept / Approach:
We test options that supply connectors “b…”, finishing with a final “b” to close “…caca” gracefully (forming the tail “cacab”). The middle joins should align with “…ab” → “…bcbc” without creating spurious duplicates.

Step-by-Step Solution:

Insert “bcab”.The first “b” after “a” builds toward “ab”.The internal “…ab bcbc…” seam reads cleanly and maintains the stepping from ab to bcbc.The tail “…caca b” produces a neat close consistent with the staircase idea.

Verification / Alternative check:
Other candidates either overuse “c” near the start or misplace letters so the transitions into “bcbc” and “caca” become clumsy.

Why Other Options Are Wrong:
ccba: Front-loads “c”, violating the a→b→c step.
acba: Inserts “a” in the middle, blurring the staircase.
bccb: Distorts the tail join with “caca”.
None of these: Not applicable because “bcab” yields consistent joins.

Common Pitfalls:
Optimizing only the front “a _ ab” join but breaking the “_ caca _” close.

Final Answer:
bcab

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