This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
Strength of sewage is best expressed by 
Select and explain. 
A. E-coli count, B. Suspended particles, C. Chemical oxygen demand, D. Biological oxygen demand
D: Biological oxygen demand
Answer: D: Biological oxygen demand 
Explanation: The most important test carried out on sewage. Sewage with a BOD value of:

300mg/I (300 ppm) or above is termed as strong. 
100 mg/I (100 ppm) or below is termed as weak.