Scour valves are provided A. At street corners to control the flow of water B. At every depression and dead ends to drain out the waste water that may collect there C. At the foot of rising main along the slope to prevent back running of water D. At every summit of rising mains

The correct answer is: C. At the foot of rising main along the slope to prevent back running of water.

Scour valves are provided at the foot of rising main along the slope to prevent back running of water. This is because when water flows down a slope, it can create a vacuum that can suck back water from the rising main. This can cause the rising main to collapse, and can also contaminate the water supply. Scour valves help to prevent this by allowing water to flow out of the rising main, even when there is a vacuum.

Option A is incorrect because scour valves are not used to control the flow of water. This is done by other devices, such as valves and gates.

Option B is incorrect because scour valves are not used to drain out waste water. This is done by other devices, such as sewers and drains.

Option D is incorrect because scour valves are not used at every summit of rising mains. They are only used at the foot of rising mains, along the slope.