Select the letter-cluster that will replace the question mark. RST, UVW, XYZ, ?

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1. Each cluster has 3 consecutive letters: RST, UVW, XYZ.
2. The starting letters are $R(18),\ U(21),\ X(24)$.
3. They rise by $+3$ each time, since $18	o21	o24$ ($24+3=27$).
4. $27$ wraps to $1$, so the next trio is **ABC**.

FINAL_ANSWER: a

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