In order for the automatic lifeboat drain to operate properly（）.
A.the cap should be removed to drain the boat when it is waterborne
B.the cage must be free of rubbish or the ball may not seat properly
C.there is an automatic ball check located in a siphon tube
D.the small lever to release the rubber ball float must be turned counterclockwise
因外伤冠折，根管治疗后要求修复，检查：见