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: Anatomy
The circle of Willis lies in: 
Select and explain. 
A. Subarachnoid space, B. Subdural space, C. Intraventricular space, D. None of the above
A: Subarachnoid space
Answer: A: Subarachnoid space 
Explanation: The circulus aeriosus (circle of Willis) is a large aerial anastomosis which unites the internal carotid and veebrobasilar systems.It lies in the subarachnoid space within the deep interpeduncular cistern, and surrounds the optic chiasma, the infundibulum and other structures of the interpeduncular fossa.Ref: Gray's anatomy 40th edition, Chapter 17.