The correct answer is A. China
Step-by-step explanation:
During the 19th century, the system of railroads in the U.S. expanded to connect regions all around the country, especially those in the West. Additionally, his expansion required thousands of workers, and this was an opportunity for immigrants as most white workers did not want to work in this area.
In this context, Chinese workers became the majority in the workforce as thousands migrated to the U.S. and they were looking for job opportunities. Indeed in some cases, Chinese workers were 90% of the workers and it is estimated around 200 thousand workers from this country contributed to the construction of railroads.