The main topic of the passage is _______.

Câu hỏi: 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.
The invention of the electric telegraph gave birth to the communications industry. Although Samuel B. Morse succeeded in making it useful in 1837, it was not until 1843 that the first telegraph line of consequence was constructed. By 1860 more than 50,000 miles of lines connected people east of the Rockies. The following year, San Francisco was added to the network.
The national telegraph network fortified the ties between East and West and contributed to the rapid expansion of the railroads by providing an efficient means to monitor schedules and routes. Furthermore, the extension of the telegraph, combined with the invention of the steam-driven rotary printing press by Richard M. Hoe in 1846, revolutionized the world of journalism. Where the business of news gathering had been dependent upon the mail and on hand-operated presses, the telegraph expanded the amount of information a newspaper could supply and allowed for more timely reporting. The establishment of the Associated Press as a central wire service in 1846 marked the advent of a new era in journalism.
The main topic of the passage is _______.
A. the history of journalism
B. the origin of the national telegraph
C. how the telegraph network contributed to the expansion of railroads
D. the contributions and development of the telegraph network