大兴安岭东西两侧的地形区名称分别是（    ）

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                                                                                                         【甲】使至塞上·王维
                                                                         单车欲问边，属国过居延。征蓬出汉塞，归雁人胡天。
                                                                         大漠孤烟直，长河落日圆。萧关逢候骑，都护在燕然。
                                                                                                         【乙】出塞·王昌龄
                                                                秦时明月汉时关，万里长征人未还。但使龙城飞将在，不教胡马度阴山。