57. 數列 $\begin{cases} \frac{1}{2}, \frac{1}{3}, \frac{2}{2}, \frac{1}{4}, \frac{2}{3}, \frac{3}{2}, \frac{1}{5}, \frac{2}{4}, \frac{3}{4}, \frac{4}{6}, \dots \end{cases}$，依此規律，試問：第 50 項是多少？
(A)$\frac{7}{5}$
(B)$\frac{5}{7}$
(C)$\frac{7}{6}$
(D)$\frac{6}{7}$

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