七世纪吐蕃赞普统一青藏高原，吐蕃是今天哪一民族的祖先（    ）

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                                                                                                                                          新晴
                                                                                                                                          刘攽
                                                                                                              青苔满地初晴后，绿树无人昼梦余。
                                                                                                              唯有南风旧相识，偷开门户又翻书。

珠江三角洲区域形成外向型经济的主要条件是（　　）