Why Did the Situation for U

Why did the situation for U.S.workers begin to change by the 1870s? ​

A) Fewer immigrants entered the country,and laborers became more in demand.
B) Technology began to replace craft skills with machines,with the result that workers were relegated to less skilled areas.
C) ​Women and children were treated with more concern than in the past.
D) ​Real wages decreased and hours worked per week increased.