21. XIAP is an inhibitor of caspases. How does it work? (A) It prevents the cleavage of prodomians from executioner caspases. (B) It has a RING domain, which blocks the active site of initiator caspases. (C) It uncoils zymogen granules before than be activated by executioner caspases. (D) It attaches ubiquitin to caspases so they get degraded before they are active.