Consider a drug which is exogenous to the body. The drug is filtered, but not secreted, reabsorbed, or metabolised by the kidneys. The drug is constantly infused in a patient to maintain a plasma concentration of 2mg/L. 24 hour urine collection shows the drug being excreted at a rate of 400μg/min. What is this patient's GFR?
A. 50ml/min
B. 100ml/min
C. 150ml/min
D. 200ml/min
E. 400ml/min