33% is the unknown concentration of the tube.
Explanation:
Concentration of stock of windex = 50%
concentration of bacteria = 500 microliter
nutrient broth = 1000 micro litre +500 microlitre of windex
concentration of bacteria is the volume of bacteria present in nutrient broth culture. 
total volume in the tube is 1000
50% is the concentration of tube of bacteria and broth before adding windex.
After adding windex of 50 % concentration and the volume is 500 microliter which is only 250 microliter of windex rest is water.
33% is the concentration of the tube, which means it has 33% of bacteria in 1500 ml of solution.
 
        
             
        
        
        
True
S phase - 
synthesis phase, during this phase , a cell copies it's DNA
        
             
        
        
        
<span>C. A greater number of wavelengths pass a given point per second.
hope this helps
</span>
        
                    
             
        
        
        
A biological community with organisms interacting within their environment :)