This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
Which differentiating prerenal azotemia with ATN features favoring pre-renal azotemia -a) Urine osmolality > 500 mosmol/kgb) Sodium spot excretion < 10 ml/Lc) Fractional excretion of sodium > 1d) Plasma BUN/creatinine ratio < 20 
Select and explain. 
A. ac, B. a, C. ab, D. bc
C: ab
Answer: C: ab 
Explanation: None