They saw how smoking effected the body since smoking was pushed as something good for a long time, a lot of people started smoking and after a few years doctors finally saw a difference between a smokers body and a non smokers body
        
             
        
        
        
The scientific method is the "process of experimentation" for your hypothesis. There are five steps in the scientific method they consist of: conduct research, form hypothesis, test hypothesis, record data, draw conclusion.
Hope this helps!
 
        
                    
             
        
        
        
Mitochondria is the powerhouse of the cell. Only thing I remember from middle school science
        
             
        
        
        
Answer:
That is administered and controlled
Step-by-step explanation:
A variable is the condition or a product that may vary without the influence of any investigator. Such variables can be found abundantly in our daily lives, ranging from air to sunlight. Controlling such variables ensures the limitation of their influence on the outcome of an experiment. In connection to this experiment, the variables that are controlled are the mineral content in the water, the  amount of bacteria food, the amount of light and the the constant temperature of 20 degrees C.
If not administered or these variables controlled, the experiment would not clearly explain the impact of caffeine on the heart beat of the water fleas as other variables may have influenced it. Thus variables are controlled to study the singular impact of any agent or effect.