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: Radiology
Which of the following is not a diagnostic feature of gallstone ileus on a plain abdominal radiograph? 
Select and explain. 
A. Ectopic calcified gallstone, B. Stone < 2.5 cm size in the intestine, C. Signs of small intestinal obstruction, D. Gas in the biliary tree
B: Stone < 2.5 cm size in the intestine
Answer: B: Stone < 2.5 cm size in the intestine 
Explanation: Characteristic feature of gallstone ileus is Rigler's triad :

Incomplete or complete small bowel obstruction; 
Gas in biliary tree; 
Ectopic gallstone.