Cross regulators in main canals are provided A. To regulate water supply in the distributaries B. To increase water head upstream when a main canal is running with low supplies C. To overflow excessive flow water D. None of these

The correct answer is A. To regulate water supply in the distributaries.

Cross regulators are structures that are built across a canal to control the flow of water. They are typically used to regulate the water supply to distributaries, which are smaller canals that branch off from the main canal. Cross regulators can also be used to control the water level in the main canal, to prevent flooding, and to improve the efficiency of irrigation.

Option B is incorrect because cross regulators do not increase the water head upstream. The water head is the pressure of the water, and it is determined by the height of the water in the canal. Cross regulators do not change the height of the water in the canal, so they do not affect the water head.

Option C is incorrect because cross regulators do not overflow excessive flow water. Cross regulators are designed to control the flow of water, not to allow it to overflow. If there is too much water in the canal, a cross regulator can be used to divert the water to another canal or to a storage reservoir.

Option D is incorrect because cross regulators are used to regulate water supply in the distributaries.