During the Industrial Revolution of the 19th century, machines took over most of the manufacturing work from men, and factories replaced craftsmen’s workshops. The event that laid the groundwork for this monumental change was the introduction of interchangeable parts, or pre-manufactured parts that were for all practical purposes identical, into the firearms industry. Interchangeable parts, popularized in America when Eli Whitney used them to assemble muskets in the first years of the 19th century, allowed relatively unskilled workers to produce large numbers of weapons quickly and at lower cost, and made repair and replacement of parts infinitely easier.
The answer for your question is D
