A group that grades the water quality of the Potomac River is giving the waterway a B. The Potomac Conservancy says the river has come a long way since earning a D for water quality in 2011. But the group noted yesterday that the river has earned a B each of the last five years, meaning progress to improve water quality has stalled. The group is trying to raise at least eight-million dollars to help pay for additional efforts to help clean up the river.