Besides Inventing the Cotton Gin, What Did Eli Whitney Do

Besides inventing the cotton gin, what did Eli Whitney do that furthered American manufacturing?

A) He developed railroad ties that were capable of supporting heavy weight.
B) He showed how water could be used to power textile mills.
C) He introduced the concept of interchangeable parts
D) He marketed American manufacturing products in Europe
E) He invented a heating process that removed impurities from iron.