What was the key to American manufacturing in the early nineteenth century?

A) The innovative financial systems that funded plant expansion.
B) The unique method of corporate management.
C) The use of machine-tooled, interchangeable parts.
D) The vertical integration of companies.

The use of machine-tooled, interchangeable parts was the key to American manufacturing in the early nineteenth century. This allowed for mass production, standardization of parts, and easier repairs and replacements. It was a major factor in the growth of American industry and the ability to compete with European manufacturers.