A rapid means of long-distance transportation became a necessity for the

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问题    A rapid means of long-distance transportation became a necessity for
the United States as settlement spread farther westward. For a long time
the railroad companies met with troubled mechanical problems. The most    【M1】______
serious were the construction of rails able to bear the load, and the
development of the safe, effective stopping system. Once these were solved,【M2】______
and the railroad was established as the best means of land transportation. 【M3】______
By 1860 they had built thousands of miles of railroads crossing the eastern
mountain ranges and reaching westward in the Mississippi.
The high point in railroad building came by the construction of the       【M4】______
first transcontinental railroad. In 1862 Congress had authorized two    【M5】______
railroad companies to build lines from Nebraska westward and from
California eastward to a meeting point so as to complete a transcontinental
system. The Central Pacific Company, starting from California, used
Chinese labor, the Union Pacific employed crews of Irish laborers.       【M6】______
The two groups worked at remarkable speed, each tried to cover a          【M7】______
greater distance than the other. In 1869, they met at a place called
Promontory. Many visitors came there for the great occasion. There
were joyous celebrations all over the country, with parades and the
ringing of church bells honor the great achievement.                      【M8】______
The railroad was very important in discouraging westward movement.       【M9】______
It also helped build up industry and farming by moving raw materials
and by distributing products rapid to distant markets.                   【M10】______ [br] 【M5】

选项

答案 had→had

解析语法错误。此句句首有表示过去的时间状语In 1862，因此这里只能用一般过去时。

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