Strength of sewage is best expressed by: (A) E-coli count, (B) Suspended particles, (C) Chemical oxygen demand, (D) Biological oxygen demand
Answer is D. 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.