已知圆中一段弧长正好等于该圆的外切正三角形的边长，那么这段弧所对的圆心角的弧度数为  (　   )

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                                                                                                 过故人庄
                                                                                                  孟浩然
                                                                    故人具鸡黍，邀我至田家。绿树村边合，青山郭外斜。
                                                                    开轩面场圃，把酒话桑麻。待到重阳日，还来就菊花。