Which one of the following statements regarding septic tanks is wrong :

• A. a gap of 7.5 cm between the bottom of the covering slab and the top level of scum is provided
• B. the outlet invert level is kept 5 to 7.5 cm below the inlet invert level
• C. the minimum width of septic level is 90 cm
• D. the depth of tank is kept equal to its width.