求翻译：The silencers are foreseen at a parity of DN (Nominal Diameter) in three lengths named 10,15 and 20 that express the following ratios是什么意思？

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The silencers are foreseen at a parity of DN (Nominal Diameter) in three lengths named 10,15 and 20 that express the following ratios

问题补充：

匿名	2013-05-23 12:21:38 沉默是预计在DN（公称直径）命名为10,15和20三种长度的奇偶，表达了以下比例

匿名	2013-05-23 12:23:18

匿名	2013-05-23 12:24:58

匿名	2013-05-23 12:26:38

匿名	2013-05-23 12:28:18

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