下列那一項為計算純水清除率（free-water clearance, \mathrm{C}_{\mathrm{H2O}}）的公式（V = urine flow rate; \mathrm{C}_{\mathrm{osm}} = osmolar clearance）？
\mathrm{C}_{\mathrm{H2O}}
\mathrm{C}_{\mathrm{osm}}
\mathrm{C}_{\mathrm{H2O}} = V + \mathrm{C}_{\mathrm{osm}}
\mathrm{C}_{\mathrm{H2O}} = V－\mathrm{C}_{\mathrm{osm}}
\mathrm{C}_{\mathrm{H2O}} = V．\mathrm{C}_{\mathrm{osm}}
\mathrm{C}_{\mathrm{H2O}} = V / \mathrm{C}_{\mathrm{osm}}
