In his transformation experiments, what phenomenon did Griffith observe?

Mixing a heat-killed pathogenic strain of bacteria with a living nonpathogenic strain converts the living cells into the pathogenic form.

Mixing heat-killed nonpathogenic bacteria with a living pathogenic strain makes the living strain nonpathogenic.

Infecting mice with nonpathogenic strains of bacteria makes them resistant to pathogenic strains.

Mice infected with a pathogenic strain of bacteria can spread the infection to other mice.