At the junction of sewers

At the junction of two sewers of different diameters, the standard principle is that the soffit (top inside surface) of the smaller sewer should not be lower than the soffit of the larger sewer. This arrangement helps maintain proper hydraulic conditions and reduces the possibility of backwater entering the smaller sewer.

Option A: Top of smaller sewer is kept lower
Incorrect. The smaller sewer's top should not be lower than the top of the larger sewer.

Option B: Top of larger sewer is kept lower
Incorrect as a general statement. The standard rule is expressed in terms of the smaller sewer's soffit being at the same level or higher than that of the larger sewer, rather than stating that the larger sewer's top is always kept lower.

Option C: Tops of both the sewers are at the same level
Incorrect. The tops may be at the same level in some arrangements, but this is not the general rule for every sewer junction.

Option D: None of these
Correct, because none of the first three options states the standard rule accurately and generally.

Hence, Option D — None of these is the correct answer.