This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urinary bladder, Subject name: Surgery
What is the composition of this stone? 
Select and explain. 
A. Calcium oxalate monohydrate, B. Calcium oxalate dihydrate, C. Ammonium urate, D. Calcium oxalate
B: Calcium oxalate dihydrate
Answer: B: Calcium oxalate dihydrate 
Explanation: - Stone has spikes. This stone is known as JACK STONE.- Most commonly found in urinary bladder- Composition is calcium oxalate dihydrate