Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.The work of the railroad pioneers in America became the basis for a great surge of railroad building halfway through the nineteenth century that linked the nation together as never before. Railroads eventually became the nation's number one transportation system and remained so until the construction of the interstate highway system halfway ...

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 36 to 42.

The work of the railroad pioneers in America became the basis for a great surge of railroad building halfway through the nineteenth century that linked the nation together as never before. Railroads eventually became the nation's number one transportation system and remained so until the construction of the interstate highway system halfway through the twentieth century. They were of crucial importance in stimulating economic expansion, but their influence reached beyond the economy and was pervasive in American society at large.

By 1804, English, as well as American inventors, had experimented with steam engines for moving land vehicles. In 1920, John Stevens ran a locomotive and cars around in a circular track on his New Jersey estate, which the public saw as an amusing toy. And in 1825, after opening a short length of a track, the Stockton to Darlington Railroad in England became the first line to carry general traffic. American business people, especially those in the Atlantic coastal region who looked for better communication with the West, quickly became interested in the English experiment. The first company in America to begin actual operations was the Baltimore and Ohio, which opened a thirteen mile length of track in 1830. It used a team of horses to pull a train of passenger carriages and freight wagons along the track, steam locomotive power didn't come into regular service until two years later.

However, for the first decade or more, there was not yet a true railroad system. Even the longest of the lines were relatively short in the 1830's, and most of them served simply to connect water routes to each other, not to link one railroad to another. Even when two lines did connect, the tracks often differed in width, so cars from one line couldn't fit onto tracks of the next line. Schedules were unreliable and wrecks were frequent. Significantly, however, some important developments during the 1830's and 1840's included the introduction of heavier iron rails, more flexible and powerful locomotives, and passenger cars were redesigned to become more stable, comfortable, and larger. By the end of 1830 only 23 miles of track had been laid in the country. But by 1936, more than 1,000 miles of track had been laid in eleven States, and within the decade, almost 3,000 miles had been constructed. By that early age, the United States had already surpassed Great Britain in railroad construction, and particularly from the mid-1860's, the late nineteenth century belonged to the railroads

The word "their" in paragraph 1 refers to_____________.