Ventilating shafts are provided to a sewer line at every A. 100 m B. 150 m C. 200 m D. 300 m

The correct answer is: C. 200 m.

Ventilating shafts are provided to a sewer line at every 200 m to provide fresh air to the sewer line and to prevent the build-up of gases. The gases in a sewer line can be harmful to human health and can also cause explosions.

Option A is incorrect because 100 m is not a common distance between ventilating shafts. Option B is incorrect because 150 m is not a common distance between ventilating shafts. Option D is incorrect because 300 m is not a common distance between ventilating shafts.