Which of the Following Best States the Relationship Between Machinery

Which of the following best states the relationship between machinery and the earnings of labor?

A) Machines tend to reduce the demand for labor, thereby reducing the earnings rate of labor.
B) Production of machinery creates jobs, thereby increasing the demand for (and wages of) labor.
C) High productivity per worker hour is a necessary ingredient for the attainment of high real earnings, and adoption of labor-saving machinery enhances the ability of labor to attain such high productivity.
D) Output and real earnings can always be increased whenever a machine can be substituted for a function previously performed by labor.