B. 15-20 years(Correct)
C. 30-40 years
D. 40-50 years
Sewage treatment units are normally designed for______________?
B. 15-20 years(Correct)
C. 30-40 years
D. 40-50 years
B. 15-20 years(Correct)
C. 30-40 years
D. 40-50 years
B. 15-20 years(Correct)
C. 30-40 years
D. 40-50 years