The design period of sewage treatment works in normally_________________?
A. 5 – 10 years
B. 15 – 20 years
C. 30 – 40 years
D. 40 – 50 years
A. 5 – 10 years
B. 15 – 20 years
C. 30 – 40 years
D. 40 – 50 years