Which of the Following Must Be True in Order to Satisfy

Which of the following must be true in order to satisfy some of Koch's postulates

A) causative agent must be observed in a small percentage of cases of disease
B) must be isolated from hosts that are not inoculated with the pure culture
C) when pure cultures inoculated in healthy hosts,they do not show disease
D) causative agent must be isolated from diseased host and grown in pure culture