In a chromatography experiment a student uses samples of three pure food dyes, blue (B), red (R) and yellow ( $$\(\mathrm{Y}\)$$ ). He also uses samples of four unknown substances, $$\(S, T, U\)$$ and $$\(V\)$$. The student puts a small drop of each substance on the pencil line. The diagram shows the student's chromatogram at the end of the experiment. Which of the unknown substances contains only one food dye? (1)

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