Passage 5
A rapid means of long-distance transportation became a necessity for the United States as settlement spread ever farther westward.The early trains were impractical curiosities,and for a long time the railroad companies met with troublesome mechanical problems.The most serious ones were the construction of rails able to bear the load,and the development of a safe,effective stopping system.Once these were solved,the railroad was established as the best means of land transportation.By 1860 there were thousands of miles of railroads crossing the eastern mountain ranges and reaching westward to the Mississippi.There were also regional southern and western lines.
The high point in railroad building came with the construction of the first transcontinental system.In 1862 Congress authorized two western railroad companies to build lines from Nebraska westward and from California eastward to a meeting point,so as to complete a transcontinental crossing linking the Atlantic seaboard with the Pacific.The Government helped the railroads generously with money and land.Actual work on this project began four years later.The Central Pacific Company,starting from California,used Chinese labor,while the Union Pacific employed crews of Irish laborers.The two groups worked at remarkable speed,each trying to cover a greater distance than the other.In 1869 they met at a place called Promontory in what is now the state of Utah.Many visitors came there for the great occasion.There were joyous celebrations all over the country,with parades and the ringing of church bells to honor the great achievement.The railroad was very important in encouraging westward movement.It also helped build up industry and farming by moving raw materials and by distributing products rapidly to distant markets.In linking towns and people to one another it helped unify the United States.
The building of the first transcontinental system（ ）.
根据文章最后一段可知该铁路把原材料及产品运往遥远的市场，对美国工农业的发展及统一起到了很大的作用，对应选项A的表达。选项BCD文中并未提到，故正确答案是选项A。
下列关于公司的表述，正确的是（）
以下各项列举了我国常见气象灾害与其发生地区的对应关系，其中正确的一项是（）
根据电视收视率调査，看体育节目的观众中青年人比中老年人要多。
由此可推断出（）
—种海洋蜗牛产生的毒素含有多种蛋白，把其中的一种给老鼠注射后，会使有两星期大或更小的老鼠陷入睡眠状态，而使大一点的老鼠躲藏起来。当老鼠受到突然的严重威胁时，非常小的那些老鼠的反应是呆住，而较大的那些老鼠会逃跑。
以上陈述的事实最有力地支持了以下哪项假说？（）
疾病控制中心对某校髙中三个年级的学生进行抽样做视力状况调查，抽样的方法为分层抽样（按比例抽样），若高中一、二、三年级学生人数分别为626、703、780，样本容量为84，则应从离二年级抽样的学生人数为多少？（）
