Answer:
External stimulus is the type of stimulus an allergen, such as pollen, is classified.
Explanation:
It is a control, which is used for the comparison between it and the other experimental groups in an experiment.
The distance from the Sun allows for the right amount of light absorption, allows the right intensity of heat, and the gravitational pull of the sun on the earth keeps it from going out into outer space.
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Saliva is made the sameway that you make tears. you have glands wich have a rich blood supply. saliva production is controlled by the autonomic nervous system, wich controls involuntary actions