Normally prestressing wires are arranged in the lower part of the beam. This is because the tensile stresses in the concrete are greatest at the bottom of the beam, and the prestressing force helps to counteract these stresses. This makes the beam more resistant to bending and cracking.
The other options are incorrect because they do not reflect the most common practice for arranging prestressing wires. Option A is incorrect because the tensile stresses in the concrete are greatest at the bottom of the beam, and the prestressing force would not be as effective in counteracting these stresses. Option B is incorrect because the prestressing force would tend to increase the tensile stresses in the concrete at the bottom of the beam. Option C is incorrect because the prestressing force would not be as effective in counteracting the tensile stresses in the concrete.
In some cases, it may be necessary to arrange the prestressing wires in a different location, such as in the center of the beam. This may be done to accommodate other design considerations, such as the need to provide access for inspection or maintenance. However, this is not the most common practice.