This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Excretory System, Subject name: Physiology
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"? 
Select and explain. 
A. +5.0 ml/min, B. +2.5 ml/min, C. 0.0 ml/min, D. -2.5 ml/min
B: +2.5 ml/min
Answer: B: +2.5 ml/min 
Explanation: Free water clearance is calculated as: urine flow rate (600 ml/2 h, or 5 ml/min) - osmolar clearance (urine osmolarity x urine flow rate/plasma osmolarity). Therefore, free water clearance is equal to +2.5 ml/min.