Fill the blanks with an ordered 4-letter set: b b _ a a b _ c a a b _ c a _.

Introduction / Context:
The alphabet subset is {a, b, c} with a strong presence of “bb”, “aa”, and “ab/ba” alternations. We must fill four blanks so the sequence keeps recurring “ab”, “bc”, and “ca” joints without introducing heavy triple runs.

Given Data / Assumptions:
Skeleton: b b _ a a b _ c a a b _ c a _ (4 blanks). Options: abbc, bcab, cbba, acab.

Concept / Approach:
We prefer a fill that (i) ends the initial “bb_aa” zone with “bc” or “ba” that blends into “…aab…”, (ii) supplies “c a” transitions, and (iii) closes “…c a _” neatly.

Step-by-Step Solution:
1) Insert “b c a b”.2) The first blank “+ b” after “bb” keeps a controlled b-cluster and leads into “…aab…”.3) The mid “+ c” and “+ a” reinforce “…b c a a b…”, a common pattern.4) The final “+ b” at the tail yields “…c a b”, maintaining the a→b progression and avoiding triple repeats.

Verification / Alternative check:
Check bigram flow: “…bb|ba/ab…”, “…ab|bc…”, “…ca|ab”.

Why Other Options Are Wrong:

• abbc / cbba / acab: Each causes at least one misaligned joint (e.g., “aac”, “cbb”, or “aca”) that contradicts the alternation implied by the skeleton.

Common Pitfalls:
Repairing the head but breaking the tail; always check all four insertions.

Final Answer:
bcab