A patient's urine is collected for 2 hours, and the total volume is 600 milliliters during this time. Her urine osmolarity is 150 mOsm/L, and her plasma osmolarity is 300 mOsm/L. What is her "free water clearance"?
+5.0 ml/min +2.5 ml/min 0.0 ml/min -2.5 ml/min