材料一  中国是世界第一大铁矿石进口国和第一大粗钢生产国，2015年粗钢产量约占全球产量的50%，但高端钢材生产不足。图17为“2015年世界铁矿石主要流向及流量略图”。

材料二  有研究显示，一个国家或地区的人均粗钢需求增幅与人均GDP有一定的相关性。2015年、2017年中国人均GDP分别为7990美元和8836美元。图18为“人均粗钢需求增幅与人均GDP关系示意图”。

材料三  产能利用率是指行业发挥生产能力的程度，是实际产量与设计产能的比率。一般认为，钢铁行业产能利用率处于78%～83%时较为合理，低于75%则为产能严重过剩。图19为“我国2010年～2015年粗钢产能和产能利用率统计图”。

古往今来的人口迁移，其最主要最根本的因素是（    ）

阅读下面语篇并根据语篇内容回答问题
    A train stopped at a small station. A man on the train looked out of the window and saw a woman. The woman was selling cakes, and the people from the strain bought them. The man wanted to buy a cake, but the woman was far away from him. It was raining, and he didn't want to go out. Suddenly he saw a boy. The boy was walking on the flatform（月台）not far from him. “Come here.” He said to the boy, “Do you know how much each of the cakes is?” “Five cents（分）.” The boy answered. The man gave the boy ten cents and asked him to buy two cakes. “One is for me and the other is for you” he told the boy. Some minutes later, the boy came back. He was eating a cake. He gave the man five cents and said, “There was only one    cake left（剩下的）.”