The answer is D, i.e., wtach to determine how long it takes to cause illness. Basically, Koch's postulates were proposed to ientify the caustaive agent of the disease. It included germ should be present in the ill perosn, germ should be isolated and grown in re culture, it must be injected into healthy animals and should be able to cause disease, the organsim must be then isolated from the injected perosn and grown in pure cultures, it should resemble the first organism, which was isolated from the ill person.
