C) They are very small uni-cellular microorganisms.
Explanation:
Microorganisms are very small organisms which are not visible to naked eyes. They include bacteria, some protists, some fungi etc. Viruses are also very small pathogens which can infect all types of living organisms, from plants to animals to bacteria.
Viruses are traditionally not considered microorganisms because they behave like non living entities outside the host cell. They do not have a proper cellular structure. Most of them just have genetic material enclosed in a protein shell. They do not even perform normal metabolic functions like replication outside the hosts's body. Hence, they are not considered microorganisms.
Animals needs oxygen's to survive and plants need carbon dioxide to survive. The ozone layer reflects harmful UV rays from the sun to protect the things living on Earth.
