Which of the following is not a diagnostic feature of gallstone ileus on a plain abdominal radiograph?: (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
Answer is B. Characteristic feature of gallstone ileus is Rigler's triad :

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