Generally the weir is aligned at right angles to the direction of the main river current because A. It ensures less length of the weir B. It gives better discharging capacity C. It is economical D. All of the above

The correct answer is D. All of the above.

A weir is a barrier constructed across a stream or river to raise the water level. Weirs are used for a variety of purposes, including irrigation, flood control, and water power generation.

The alignment of a weir is important because it affects the efficiency of the weir. A weir that is aligned at right angles to the direction of the main river current will have a better discharging capacity than a weir that is aligned parallel to the current. This is because the water will flow over the weir more easily when it is aligned at right angles to the current.

In addition, a weir that is aligned at right angles to the current will be more economical to construct. This is because the weir will require less material to build.

Therefore, the correct answer is D. All of the above.