You are presented with the question "Dead tissue can be caused by what?" and the following answer choices: 
- disease
 - pollen counts
 - advanced mathematics
 - microorganisms
 - migrating birds
 - viruses
 - Physical weathering
 - harmful substances

Now knowing that microorganisms can cause infections and leeches clean away the dead or infected tissue, choose the best answer.


 

    microorganisms     