The correct answer is: Even though the S strain had been heat-killed, it changed the R strain.
Griffith in his experiment used two strains of bacteria:
• The rough strain (R) which did not cause pneumonia, (nonvirulent)
• the smooth strain (S) which did caused pneumonia (virulent)
When Griffith heat-killed the S strain it did not caused mice to die.
But, when he gave both the heat-killed S strain and the R strain to mice, the mice developed pneumonia and died.
Griffith conclusion was that the R-strain bacteria took what he called a "transforming principle" from the heat-killed S bacteria which transformed the nonvirulent R strain.
