Answer
If neither radioactive substances were used as markers, neither the presence of DNA nor the protein would have been discovered, and they would have been unable to make a scientific conclusion.
Work Step by Step
DNA contains no sulfur, which means that if were sulfur to appear in the bacteria's molecules, it would be a result of the virus having injected proteins into the bacteria. On the contrary, were phosphorus to appear, it would mean that DNA had been injected into the bacteria, because proteins do not carry a significant amount of phosphorus.
