Refer the given flowchart of sewage treatment, accordingly match Column I with Column II and select the correct answer from the codes given below:


$\begin{array}{cccc}& \text{Column I}& & \text{Column II}\\ \text{A.}& \text{The stage in which physical}\text{treament of sewage is done}& \text{(i)}& \text{Anaerobic degestion of activated}\text{sludge and production of}\text{biogas}\\ \text{B.}& \text{The stage in which biological}\text{treatment of sewage is done}& \text{(ii)}& \text{Activated sludge}\\ \text{C.}& \text{Name of the sediment in primary treatment}& \text{(iii)}& \text{Aeration tanks}\\ \text{D.}& \text{It is carried to aeration tanks from}\text{primary settling}& \text{(iv)}& \text{Primary effluent}\\ \text{E.}& \text{Name of the sediment in}\text{secondary treatment}& \text{(v)}& \text{Primary sludge}\\ \text{F}& \text{Site of flocs growth}& \text{(vi)}& \text{Secondary treatment}\\ \text{G}& \text{Function of sludge digester}& \text{(vii)}& \text{Primary}\end{array}$

The correct option is A
A–(vii), B–(vi), C–(v), D–(iv), E–(ii), F–(iii), G–(i)
Primary treatment removes floating and suspended solids from sewage through filtration and sedimentation.
Secondary treatment is purely a biological treatment involving microbial activity.
Most of the suspended material get concentrated and settles down. The sedimentation is called the primary sludge and supernatant is called effluent.
The effluent is passed through a sedimentation tank where microbial flocs are allowed to settle down. The settled material is called activated sludge. A part of activated sludge is used as in column in aeration tank.
The remaining is passed into a large tank called anaerobic sludge digester.
Therefore, the correct option is A.