Which of the following arguments is consistent with the claims of Robe

Which of the following arguments is consistent with the claims of Robert Fogel (1989)?

(a) The success of slave labor was tied closely to the number of labor hours per year.
(b) Compared to the slaves, the self-sufficient farmers in the North and those in the South were more likely to not work on Sundays, face a shorter work day and experience regular rest times.
(c) The productivity of slave labor increased with the use of specialization and rhythm.
(d) All of the above.

ANSWER

(c)