You can be infected with a virus & not even know it & that is true
Animals use the respiratory system to breathe in oxygen and other gases and expel gases like CO2, right? O2 in, CO2 out.
The answer to your question is <span>B. Natural selection acted on the variation that was present in the population.
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There are two types of biological organization, I believe.
The first would be (from smallest to largest).
Species
Genus
Family
Order
Class
Phylum
Kingdom
Domain
The other would be:
Atom
Molecule
Cell
Tissue
Organ
Body system
Organism (Individual)
Population
Community
Ecosystem
Biosphere
Hope this helps c: