18．递等式计算．（能简算的要简算）
| $1\frac{5}{7}-\frac{5}{12}-\frac{7}{12}$ | $2-（\frac{1}{5}-\frac{1}{6}）$ | $\frac{5}{8}-\frac{9}{17}+\frac{3}{8}-\frac{8}{17}$ |
| $\frac{5}{9}+\frac{2}{3}-\frac{7}{18}$ | $\frac{4}{5}-（\frac{3}{5}-\frac{1}{3}）$ | $\frac{7}{8}+\frac{1}{6}-\frac{13}{24}$ |
16．下面（ ）号图是正方体的展开图．
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| A． |  | B． |  | C． |  |