如图，要把池中的水引到D处， 可过D点引DC⊥AB于C，然后沿DC开渠，可使所开渠道最短，试说明设计的依据：____ ．

阅读下面的诗歌，完成后面的题目。

                                                                                                        江 上

                                                                                                 （宋•王安石）
                                                                                                 江北秋阴一半开，

                                                                                                 晚云含雨却低徊。

                                                                                                 青山缭绕疑无路，

                                                                                                 忽见千帆隐映来。