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: Pathology
Serum acid phosphatase is increased in: 
Select and explain. 
A. Paget's disease., B. Osteopetrosis., C. Osteogenesis imperfecta., D. None.
B: Osteopetrosis.
Answer: B: Osteopetrosis. 
Explanation: None