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Algebra Difficulty 2.0 Junior Find the answer United Kingdom

Sanja has two bowls of numbered balls. Bowl X contains seven balls numbered 1, 2, 6, 7, 10, 11 and 12. Bowl Y contains five balls numbered 3, 4, 5, 8, and 9. Which ball should Sanja transfer from Bowl X to Bowl Y to increase the mean of the numbers on the balls in each bowl?

(A) $12$ (B) $11$ (C) $10$ (D) $7$ (E) $6$

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