Planets sometimes seen to reverse direction in their course across the sky with respect to the
distant stars. This effect, known as retrograde
motion, is caused by

clinically, some people often have repeated allergies, eczema and respiratory infections or other parts of the infection, in this case, in addition to some routine tests such as tests for white blood cells, immunoglobulin series, lymphocyte subsets, etc., the five immune immunoglobulin tests are also necessary.

the correct answer is:

a population changes in response to a change in the environment.

looks like the answer is     c     to me!

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