How does the article develop the idea of the railroads' importance in 19th-century United States?

A
by describing the labor that was involved in the completion of the transcontinental rail link
B
by giving the date of the first American railroad and explaining that it had 13 miles of track
C
by explaining that railroads would not have been possible without the invention of steam power
D
by describing how the railroads grew quickly and demonstrated the nation's technological strength
please help its on newsela if you tell me wrong answer it will lower my grade.