The angle subtended by the surface of sewer water with partial flow, at sewer centre is 120°, the depth of sewerage is A. 20 cm B. 25 cm C. 40 cm D. 50 cm

The correct answer is $\boxed{\text{A}}$.

The angle subtended by the surface of sewer water with partial flow, at sewer centre is 120°. This means that the sewer is 120° wide at the surface. If we assume that the sewer is a perfect circle, then the depth of the sewer is half of the width, or 20 cm.

Option B is incorrect because it is the depth of the sewer if it were a perfect square. Option C is incorrect because it is the depth of the sewer if it were a perfect triangle. Option D is incorrect because it is the depth of the sewer if it were a perfect rectangle.