2025 CSAT — Q30

The pattern is the repeating block ABBCCC (one A, two B’s, three C’s), laid down end to end. The string $\text{AB\_CC\_A\_BCCC\_BBC\_C}$ has exactly $18$ characters — three full blocks of six — so the tiling is forced. Mark off the blocks and read which letter each blank must carry.

Write the string with the five blanks numbered:

• Block 1 $\text{A B \_ C C \_}$ must read A B B C C C $\Rightarrow$ blank $1=\text{B}$, blank $2=\text{C}$.
• Block 2 $\text{A \_ B C C C}$ must read A B B C C C $\Rightarrow$ blank $3=\text{B}$.
• Block 3 $\text{\_ B B C \_ C}$ is the third ABBCCC block (it starts at character $13 = 2\times6+1$, exactly a block boundary), so it must read A B B C C C $\Rightarrow$ blank $4=\text{A}$ (the block-opening A), blank $5=\text{C}$.

Reading the blanks in order: $\text{B, C, B, A, C}$.

Answer: (c) B, C, B, A, C.

Letter-block completion is won by finding the repeating unit first, then treating every blank as “which letter finishes this block.” The unit here is ABBCCC (counts $1,2,3$); once you see it, blanks $1$–$3$ are forced immediately. The decisive check is the third block’s opening: character $13$ is a block boundary ($2\times6+1$), so it must be the block-opening A — this is exactly what separates the answer $\text{(c)}$ from the near-miss $\text{(a)}$, which swaps the last two fills. The engineered distractors each break a different structural rule — the one-A-per-block count, the block boundary, or the unit itself — so verifying that your fill reproduces a clean ABBCCC | ABBCCC | ABBCCC tiling is the discipline that scores.