The correct option is A. The slow progress during the contruction of the Transcontinental Railroad was caused by the hazardous mountain regions made work difficult.
The construction of a transcontinental railroad was one of the United States' major technological achievements. Railroad track had to be located over 2,000 miles of rugged terrain, including mountains of solid granite. The progress in the tunnels through the mountains was agonizingly slow, an average of a foot a day.
