Which of the following is an advantage of separate sewers?

• In separate sewer systems, as two sets of sewers are used, the quantity of wastewater flow will also be less in sewers. Hence, the size of the sewers will be small. 
• As two sewers are used, it doesn’t prove to be economical.
• It provides the same waste quantity, So, it does not reduce the waste.
• River pollution can not be considered a disadvantage.  

Hence, "The size of the sewers is small" is considered the correct answer.