Each set \( X_{r} \) contains 5 elements and each set \( Y_{r} \) contains 2 elements and \( \bi...

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Each set \( X_{r} \) contains 5 elements and each set \( Y_{r} \) contains 2 elements and \( \bigcup_{r=1}^{20} X_{r}=S=\bigcup_{r=1}^{n} Y_{r} \). If each element of \( S \) belongs to exactly 10 of the \( X_{r}{ }^{\prime s} \) and to exactly 4 of the \( Y_{r}^{\text {'s }} \), then \( n \) is
(A) 10
(B) 20
(C) 100
(D) 50
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