Answer: C. Sill
The top of the weir is called Sill.
A Weir is a structure which is constructed across a river or canal. Weir is constructed to store the water on the upstream side of the river or canal.
Nappe or vein is the layer of flowing water over the weir. The top of the weir over which water is flowed is called sill or crest of weir
The height above the bed of the channel is called the crest height. Hence, the top of the weir is called sill.
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