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: Radiology
"Bone within a Bone" appearance is seen in: 
Select and explain. 
A. Osteogenesis imperfecta, B. Osteopetrosis, C. Scurvy, D. Rickets
B: Osteopetrosis
Answer: B: Osteopetrosis 
Explanation: Osteopetrosis