B第二级阶梯，一二阶梯的分界线是昆仑山，祁连山，横断山脉，二三阶梯分界线是大兴安岭，太行山，巫山，雪峰山
http://image.baidu.com/i?ct=503316480&z=0&tn=baiduimagedetail&word=%C8%FD%B4%F3%BD%D7%CC%DD&in=32763&cl=2&cm=1&sc=0&lm=-1&pn=0&rn=1&di=345663092&ln=4故选B
B
