Fact 1: Viruses are not cells.

Fact 2: Viruses Viruses are difficult to classify.

Given the two facts above, answer the question "Difficult things to classify are what?" with the following options: 
- frontal zones
 - Pollution
 - not cells
 - Allergies
 - bacteria
 - Leptospirosis
 - rays or beams
 - Pesticides


 

   not cells      