A boat pilot intends to cross a river of width \(D\) such that the boat travels along a straight-line path perfectly perpendicular to the riverbanks. The river flows with a constant speed \(v_w\) relative to the banks. The boat moves with a constant speed \(v_b\) relative to the water, where \(v_b > v_w\). To achieve the desired path, the pilot aims the boat at an angle upstream as shown in the diagram. Which of the following expressions represents the magnitude of the boat’s velocity relative to the riverbank?