I need to disconnect my laundry room tub/sink so I can level and finish the flooring. I went to disconnect and there is a 3rd small copper pipe going up into the tap. I am used to only seeing two (hot and cold) connections. What could this third small copper pipe be in the middle? It runs from the tap down into my basement wall. Its very small pipe. I have attached a picture showing the pipe in between the hot and cold inputs. It is proven very hard to access the nut to get this off and I have just debated cutting it but I have no idea what purpose it is. Can anyone help? Thanks.
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