Graphite Is Identified in a Particular Schist

Graphite is identified in a particular schist. Which one of the following conclusions is justified?

A) The schist formed from a quartz- rich, sedimentary limestone.
B) The rock also contains diamonds; both are crystalline forms of the element carbon.
C) The graphite lubricated shearing movements along a fault, causing a schist to form.
D) The pre- metamorphic rock was a shale or mudstone containing organic matter.