Hughes and Cain (2011) give some credit to which of the following factors for the 1860–1910 increase in the number of people employed, shorter work days and higher real incomes?

(a) A decrease in the number of immigrants
(b) A closed economy with no imports coming into or exports going out of the U.S.
(c) Mechanical power and capital accumulation
(d) All of the above

(c)