Which of the following most accurately explains how the use of robots and other labor-saving machines impact the living standards of people?

a. They are harmful because their use puts people out of work.
b. They are helpful because they improve worker productivity.
c. They are harmful because they are accident prone and therefore endanger the health and welfare of employees working with them.
d. They are helpful to employers, but they are harmful to both consumers and workers because they reduce overall output.