When JWilliam Schopf Observed Chains and Clumps That Resembled Modern Cyanobacteria

When J.William Schopf observed chains and clumps that resembled modern cyanobacteria in Australian sedimentary rocks, he concluded that these were remains of living organisms because

A) the structures were near-perfect spheres, a form rarely found in nature.
B) the structures had an equal ratio of l- and d-amino acid isomers.
C) the structures had internal substructures that are characteristic of living systems.
D) the structures were in sediments deposited far from water.
E) the sediments indicated that oxygen had been consumed.