Hay fever is caused by 
one celled organisms bacteria viruses chemical messengers carbon heat energy plant reproduction single-celled organisms
A B C D E F G H
Hay fever is caused by an allergy to pollens or moulds.
Hay fever is caused by plant reproduction
Hay fever is caused by  (A) one celled organisms (B) bacteria (C) viruses (D) chemical messengers (E) carbon (F) heat energy (G) plant reproduction (H) single-celled organisms