You are presented with the question "Hay fever is caused by " and the following answer choices: 
- one celled organisms
 - bacteria
 - viruses
 - chemical messengers
 - carbon
 - heat energy
 - plant reproduction
 - single-celled organisms

Now knowing that plant reproduction often requires pollen and hay fever is caused by an allergy to pollens or moulds, choose the best answer.


 

       plant reproduction  