Some methods of determining the size of population are direct and indirect observations, sampling, and mark-and-recapture studies
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Sampling - Done by counting the animals in a small area and then estimating how many are in the larger area. You go out and take samples from different places so you can take the average. In most cases, it is not possible to count every member of a population so an estimate is made. 
Mark and recapture - Another estimating method is a technique called "mark and recapture." This technique gets its name because some animals are first captured, marked, and released into the environment. Then another group of animals is captured. The number of marked animals in this second group indicates the population size.
Indirect observation - The members of a population may be small or hard to find. It may be easier to observe their tracks or other signs rather than the organisms themselves.
Direct observation - The most obvious way to determine the size of a population is to count, one by one.
 
        
                    
             
        
        
        
Answer:
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Explanation:
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Answer: atomic mass is the amount of protons and neutrons
Explanation:
 
        
             
        
        
        
The main function of white blood cells is to help protect the human body from infection as well as other foreign materials. White blood cells are also known as leukocytes, and they develop in bone marrow from stem cells. There are five different types of white blood cells<span>, including neutrophils, lymphocytes, monocytes, eosinophils and basophils.</span>
        
             
        
        
        
The spinal cord and the brain make up the central nervous system, and spinal cord nerves carry most messages between the brain and the rest of the body. 
The vagus nerve carries the signal from the brain to the heart.