You are presented with the question "Difficult things to classify are what?" and the following answer choices: 
- frontal zones
 - Pollution
 - not cells
 - Allergies
 - bacteria
 - Leptospirosis
 - rays or beams
 - Pesticides

Now knowing that viruses are not cells and viruses Viruses are difficult to classify, choose the best answer.


 

   not cells      