Some additional explanation from a researcher friend of mine at Stanford: After reading the evidence presented in the passage I would conclude that the correct answer is (B). The presence of radioactive phosphate in the cell pellet clearly suggests that phage DNA entered the bacterial cells. However, it is unlikely, given the experimental design and reported results, that phage protein was also injected into the cells. Here’s why: having done thousands of centrifugations, no separation/fractionation/centrifugation step you ever do in the lab will have 100% efficiency. During the blending step, for example, a small percentage of the T2 virus or viral protein will remain adhered to the outside of the bacterial cells. Similarly, even a small amount of completely unbound virus can contaminate the cell pellet after centrifugation. Thus, there are two steps in this protocol in which you will introduce a small amount of experimental noise. Given that the radioactive sulfur was found ‘almost exclusively’ in the supernatant, I find it highly unlikely that any was actually injected into the cells. From the presented evidence, I would not be able to conclude that phage protein ever entered the cells, and believe (B) is the most suitable answer.