water treatment

In rapid sand filter backwashing process shown in the figure below, the shut and open valves are: wasn water storage V From settling tank B) Sand Gravels de D Underdrains Select one: A. Close A and B, open C and D B. Close AB and C, Open D C. Close A and C, open Band D D. Close B and D. open A and C Which of these remove coarse materials? ? Select one: A. Grit chamber B. Bar screen C. Rapid sand filter D. Primary clarifier