I can't get this wrong. These parts are made from pieces that are exactly the same:

A. interchangeable parts

B. steam Engines

C. mass production

D. railroad ties

I gonna have to go with A. There's hundreds of different types of steam engines and mass production is simply the mass production and a good and railroad ties ... it just didn't make sence to me because there are many different materials you can use that will serve as a railroad tie.