Answer:
D) all of the above
Step-by-step explanation:
Probably the single most labor reducing factor is new technology and how it is applied within a business. Automation is probably the single most important labor reducing factor in the US over the last 30 years. This is specially for factory workers, since automation is responsible for fewer industry jobs, not China.
New technologies increase marginal returns for labor and they also have changed organizations completely.
Computers, the internet, smartphones, Amazon, etc., have changed our world. Even the military has changed, a pilot can be located inside a US base and his airplane is flying a mission in the Middle East.