Because if there is a minimal variation of it the normal state of pH, it can cause severe effects in the brain, arteries, the heart, and other organs. 
In other words, it can have some serious effects on the body systems that can lead big diseases, like cancer. 
        
             
        
        
        
The best answer to the question above would be letter d. The sentence presented which is 'Researchers helped further the biological perspective when they demonstrated that electrical stimulation of the brain could evoke emotional responses in animals.' demonstrates how new discoveries can influence perspectives. 
        
             
        
        
        
Answer:
What is the name of this website 
or the book?