Although Horses Were Once a Very Important Factor of Production

Although horses were once a very important factor of production, the development of automobiles and other machinery reduced the demand for their use. This is an example of:

A) technological progress shifting the demand for horses to the left.
B) higher marginal costs for horses shifting demand for horses to the right.
C) technological progress shifting the demand for horses to the right.
D) higher marginal costs for horses having no impact on the demand for horses.