In the experiments that led Fredrick Griffith to discover the "Transforming Principle", which of the following was the key step in illustrating that a substance within one cell could transform another cell?

A. Live strains of pathogenic and non-pathogenic bacteria were independently injected into mice.

B. Heat killed and fragmented pathogenic bacteria cells were injected into mice.

C. Heat killed and fragmented pathogenic bacteria cells were mixed with living non-pathogenic bacterial cells and then injected into mice. 

D. Live strains of pathogenic and non-pathogenic bacteria were varied together and injected into mice.