Fields Can Be Ploughed with Human Labor, Animal Power, or Fuel-Burning

Fields can be ploughed with human labor, animal power, or fuel-burning machines. Explain how the choice among the methods of ploughing depends on worker wages.

A) Human ploughing is most intense in human effort and mechanized ploughing is least intense.
B) Human ploughing is optimal if wages are very low.
C) Mechanized ploughing is optimal if wages are very high.
D) All of the above.