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: Pediatrics
A very preterm baby on 30 mL/kg of enteral feeding developed sudden severe abdominal distension with visible bowel loops on day 6 of life. The baby also showed temperature instability and lethargy. s-ray of the abdomen showed poal venous gas. The staging of NEC is: 
Select and explain. 
A. lb, B. 2a, C. 2b, D. 3a
C: 2b
Answer: C: 2b 
Explanation: Ans. c. 2b