Reference point .................................
 
        
                    
             
        
        
        
A variation is a reference to the differences or deviations, such as the structure and form, as for crossing over is the exchange of genetic material between the non-sister chromatids of the homologous chromosomes during meiosis.
        
                    
             
        
        
        
Answer:
 pfu/ml
 pfu/ml
Explanation:
Given - 
Bacteriophage dilution  ml
 ml
Volume of bacteria and plate with top agar   ml
 ml
Number of plaques  pfu
 pfu
Dilution (d) 
Final concentration of phage stock is equal to 

Where pfu is the count of plaques
d is the dilution and v is the volume
Substituting the given values, we get - 
 pfu/ml
 pfu/ml