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: Surgery
When patient is asked to look down, his upper eyelid fails to follow the rotation of the eyeball and thus lags behind, this eye sign is known as: 
Select and explain. 
A. Von Graffe's sign., B. Joffroy's sign., C. Moebius sign., D. Stellwag'sign.
A: Von Graffe's sign.
Answer: A: Von Graffe's sign. 
Explanation: None