The screens are fixed A. Perpendicular to the direction of flow B. Parallel to the direction of flow C. At an angle 30° to 60° to the direction of flow D. None of these

The correct answer is: A. Perpendicular to the direction of flow.

The screens are fixed perpendicular to the direction of flow to prevent the flow of water from eroding the soil. The screens are also designed to allow water to flow through them, which helps to prevent flooding.

Option B is incorrect because the screens are not parallel to the direction of flow. Option C is incorrect because the screens are not at an angle of 30° to 60° to the direction of flow. Option D is incorrect because the screens are not fixed at any of the angles listed.