Which of the following is the most important element of Koch’s germ theory of disease? The animal shows disease symptoms when ( ) A: the animal has been in contact with a sick animal. B: the animal has a lowered resistance. C: a microorganism is observed in the animal. D: a microorganism is inoculated into the animal.

There are 10 Escherichia coli inoculated. Escherichia coli have a generation time of 20 minutes. After a lag of 20 minutes, the E.coli enter the logarithmic phase of growth. After 3 hours of growth, the total number of cells is A: 2560 B: 5012 C: 90 D: 1028 E: 1000000