往往成为我国大地形区分界线的地形类型是（    ）

                                                                                                      登楼
                                                                                                     杜甫
                                                                             花近高楼伤客心，万方多难此登临。
                                                                             锦江春色来天地，玉垒浮云变古今。
                                                                             北极朝廷终不改，西山寇盗莫相侵。
                                                                             可怜后主还祠庙，日暮聊为梁父吟。