For a given discharge, the efficiency of sedimentation tank can be increased by______________?

A. the time taken by rainfall water to run from most distant point of water shed to the inlet of sewer
B. the time required for flow of water in sewer to the point under consideration
C. sum of (A) and (B)
D. difference of (A) and (B)

A. separate system
B. combined system
C. partially separate system
D. partially combined system

A. suspended impurities
B. dissolved impurities
C. both suspended and dissolved impuri¬ties
D. none of the above