Which step of Koch's postulates provides evidence for a cause-effect relationship between the suspected pathogen and a disease

A. Step 1 - suspected pathogen only found in diseased individuals, not in healthy people
B. Step 4 - isolate suspected pathogen from the individual in step 3; needs to be the same microbe as what was isolated in step 2
C. Step 2 - isolate suspected pathogen from host and establish a pure culture
D. Step 3 - inoculate a healthy susceptible host with the suspected pathogen and observe host for onset of disease signs and symptoms