If D.O. concentration falls down to zero in any natural drainage, it indicates the zone of

A. Fit for discharge into any open drain B. Foul and contains dissolved and suspended solids C. As good as that from a complete treatment D. None of these

A. 0°C B. 15°C C. 20°C D. 25°C

A. 1 in 5 B. 1 in 10 C. 1 in 15 D. 1 in 20

A. Without entry ladders B. Without manhole covers C. With depths more than 3.5 m D. Having drains at different levels