Fredrick Griffith published a series of experiments that collectively were dubbed "The Transforming Principle" in 1928. Which of the following stopped him from being able to declare conclusively that nucleic acid (DNA) is the transforming agent?

A. The nucleic acid and lipid fractions of the pathogenic cells were both capable to transform living nonpathogenic bacterial strains.

B. The three major cellular fractions (nucleic acid, protein and lipid) were not individually mixed with the living non-pathogenic bacterial cell population.

C. The nucleic acid, protein and lipid fractions were not radioactively labeled so it was not possible to recognize which components had been transformed.

D. The heat killed/fragmented pathogenic cells could, on their own, infect and kill a mouse so it was not clear if the non-pathogenic strains were being transformed or not.