In 1839, the Silurian System Was Defined Primarily on the Basis

In 1839, the Silurian System was defined primarily on the basis of

A) the nature of sedimentary rocks lying directly upon ancient crystalline rocks.
B) seven distinct fossil mammalian faunas of the Paris Basin.
C) expansion of glaciers at the south pole, which was related to a mass extinction.
D) trilobites, brachiopods, and crinoids, and as well as corals that formed small reefs.