How Did Robert Koch Determine That It Was Bacteria That

How did Robert Koch determine that it was bacteria that caused the disease anthrax?

A) Everybody at the time knew anthrax was caused by bacteria.
B) He injected mice with anthrax infected material, and they all came down with anthrax, while those injected with healthy material did not.
C) He put mice in close proximity to healthy and to anthrax-infected animals and only those near the sick animals contracted anthrax.
D) He photographed the anthrax bacteria.
E) He infected himself with anthrax bacteria and contracted the disease.