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: Gynaecology & Obstetrics
In a pregnant woman of 28 weeks gestation IUD is earliest demonstrated on X-ray by: 
Select and explain. 
A. Increased flexion, B. Overlapping of cranial bone, C. Spalding's sign, D. Gas in vessels
D: Gas in vessels
Answer: D: Gas in vessels 
Explanation: None