Population density is the characteristics of a population increase with plentiful of resources. Population density is the measurement of every person in the population per unit area or unit volume. It is applied to a living organism and most of the time on humans. It is also the number of people living in each unit of area such as a square mile. 
 
        
             
        
        
        
Answer:
Animals are born knowing how to do something instinctual, but they must be taught learned behaviors and practice them to get good at them. Male birds of paradise are known for having bright markings that they flash while making complex movements when females are nearby. ... This is an example of courtship behavior.
 
        
             
        
        
        
Answer:
D, organisms that survive long enough to reproduce.
Explanation:
C and D are the best answers. Adaptations and skills other than speed and strength contribute to an organism's survival too, so that means it isn't C. Natural Selection is about how an organism must have useful traits to survive to reproductive stage and  successfully reproduce, so it would definitely be D.
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Refraction is the bending of light (it also happens with sound, water and other waves) as it passes from one transparent substance into another. This bending by refraction makes it possible for us to have lenses, magnifying glasses, prisms and rainbows.
Here you go, and i hope you have a good day too :)