Answer:
They produce or process nutrients in the digestive tracts of humans and other animals.
Explanation:
 
        
             
        
        
        
Answer:
stimulus, control center
Explanation:
The control center is the main nervous system of the brain that is used to carry out different sensory, the motor and integration of data.
The stimulus control is the behavioral control that is described as the situations where behavior is triggered by the presence or the absence of any stimulus. Stimulus is the functional reaction in a tissue or any organ.
In the context, the nerve cells in the stomach responds to the stimulus that results from the foods after eating food is a mechanism between the stimulus and the control center.
 
        
             
        
        
        
Opportunistic infections are infections that occur more frequently and are more severe in people with weakened immune systems, including people with HIV.
        
             
        
        
        
Answer:
Diversifying or disruptive selection increases genetic variance when natural selection selects for two or more extreme phenotypes that each have specific advantages.