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)$\frac{6}{7}$