Big hint: This object is in the Smithsonian's National Museum of American History in Washington, D.C.

A. A model of a coin-stamping machine designed by Leonardo da Vinci

B. An "effortless" drill press designed by Benjamin Franklin

C. A copy of the works from Thomas Jefferson's "automatic" dumb waiter at Monticello

D. The interior mechanism from a seventeenth-century "tower" clock