Fact 1: Plant reproduction often requires pollen.

Fact 2: Hay fever is caused by an allergy to pollens or moulds.

Given the two facts above, answer the question "Hay fever is caused by " with the following options: 
- one celled organisms
 - bacteria
 - viruses
 - chemical messengers
 - carbon
 - heat energy
 - plant reproduction
 - single-celled organisms


 

       plant reproduction  