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: Dental
Long depression or valley between ridges and cusps, the inclines of which meet at an angle is known as: 
Select and explain. 
A. Sulcus., B. Groove., C. Lobe., D. Fossa.
A: Sulcus.
Answer: A: Sulcus. 
Explanation: None