Question

what were the two strains of bacteria griffith was experimenting with?
harmless strain and flu - causing strain
harmless strain and pneumonia - causing strain
harmful strain and virus - causing strain

Explanation:

Griffith's experiment was about bacteria related to pneumonia. He used a harmless (R - strain, rough - coated) and a pneumonia - causing (S - strain, smooth - coated) strain. The flu - causing and virus - causing strains are not relevant here as the experiment was focused on pneumonia - related bacteria.

Answer:

harmless strain and pneumonia - causing strain