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
Circulus arteriosus is formed by 
Select and explain. 
A. Anterior communicating artery., B. Basilar artery., C. Anterior, posterior cerebral and posterior communicating artery., D. All of the above.
D: All of the above.
Answer: D: All of the above. 
Explanation: Circulus arteriosus or "circle of Willis" is a anatomical peculiarity of cerebral arteries.
It is formed by
• Anteriorly by anterior communicating artery.
• Posteriorly by basilar artery.
• On each side by anterior cerebral, posterior cerebral, posterior communicating and internal carotid arteries.