Natural selection could not occur without environment, and a act on animal.
Answer: It is the key to blocking infections
Explanation: I hope this is right :)
<span>(</span><span>A. specialized cells that perform specific functions)</span><span>
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I'd say zoology.
Zoology is the study of animals, generally speaking, and since birds are animals, the study of these fall into the field zoology. The study of birds though, is called ornithology.
Hope it helped,
BioTeacher101