Answer:
amino acids in protein also have Nitrogen atoms, therefore it will be difficult for the radioactivity to differentiate between protein and DNA.
Explanation.
Possibly the students did not reasoned prior to the experimental set up that the targeted labeled atom in DNA,which is Nitrogen , is also one of the elements that forms the amino group of the , amino acids units of proteins. Therefore labeling this will create confusion in the results because the objective of the set up was to differentiate between DNA and protein.
However since nitrogen is common to the two, the radioactivity will not differentiate between the nitrogen of DNA and protein. Thus confusing results