Answer:
A is the best answer
Step-by-step explanation:
Economic and productivity growth in the United States during the late nineteenth century is explained best by the advancement of technology at the time, represented by innovations like the railroad, the assembly line, electricity, etc.
These innovations were applied by private entrepreneurs in private firms, which lead to a boost in productivity and growth, and which attracted millions of immigrants to the United States during this time, who were seeking better economic opportunities.