Answer:
Scientist count the world humans population with an organization called The United Nations Population Division (UNPD), 
Explanation:
Which keeps track of the world population, projects that the world's human population will hit 7 billion on Halloween Day 2011. Admittedly, that is just an estimate: There's no way to know exactly how many people are alive at any given moment, and the true date that humanity's ranks will surpass 7 billion could come in the weeks or months before or after Oct. 31. Nonetheless, the UN's guess is the best there is.
 
        
             
        
        
        
A: Energy hope this helps
        
             
        
        
        
<span>C. population growth begins to slow down</span>
        
             
        
        
        
He is most likely to suffer some loss of visual perception.