An activated sludge tank is 30 m and 9 m wide and has liquid depth of 4 m.The influent sewage flow rate is 3.5 MLD .The primary effluent has BOD of 130 $$\frac{mg}{l}$$ and suspended load of 15 $$\frac{mg}{l}$$. The MLSS concentration in the aeration tank is 1800 $$\frac{mg}{l}$$. The corressponding $$\frac{F}{M}$$ ratio is

A

0.19

B

0.23

C

0.32

D

0.46