患者男，36岁，3个月前因外伤一上前牙脱落，今要求烤瓷修复。检查：缺失，间隙正常，牙槽嵴无明显吸收。
牙冠1/2缺损，已露髓，探稍敏感，叩诊阴性，无松动。
牙冠良好，叩诊阴性，无松动。上下前牙牙龈轻度红肿，易出血，可见菌斑及牙石。余牙未见异常。
下列哪项对桩核牙体预备的描述是正确的（）
A．按金属烤瓷冠预备体的要求进行的残冠磨除
B．齐龈磨除的残冠
C．牙体预备不应磨除薄壁
D．为增强固位可在根管内壁预备倒凹
E．以上都不对
参考答案：A
解析：
桩核的牙体预备应首先按照外冠修复体对预备体的要求磨除剩余牙体组织，然后去除薄壁、龋坏组织等，应尽量保留牙冠的硬组织。残冠预备完后进行根管的预备，根管内壁应平滑、无倒凹。因此以上选择中只有A是正确的。
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| Bobby was getting cold in the snow. The thin 1 he wore had a few holes in them and people could 2 his toes (脚趾). Bobby had been there for about an hour already. 3 he tried, he still didn't have any 4 of what he could give his mother as a Christmas gift. He didn't have any money. Bobby's mother 5 at night in the hospital, but the little money she made just 6 seemed to be enough. The family was always short of money and material things. 7 , it was never lacking in love. Both Bobby's elder sisters had made beautiful gifts for their 8 . It was Christmas Eve already, but Bobby had 9 . Then suddenly his 10 were caught by something on the road. It was a shiny coin. Never before had Bobby felt he was so 11 . Bobby went into the shop, but his 12 quickly turned cold when a salesgirl told him that he 13 buy nothing with only one coin. Then he noticed a 14 shop. The shop owner asked Bobby if he could 15 him. Bobby gave him the coin and asked if he could but one rose for his mother. The shop owner put his hand on Bobby's shoulder and 16 , "I just happened to have some roses on 17 for a coin of a dozen. Would you like them?" Bobby was 18 . Could this be true? Minutes 19 , the shop owner came to Bobby with a box of twelve beautiful red roses. Bobby knew it was true. He 20 the shop owner, and then left with the beautiful flowers. He didn't feel cold at all. | ||||
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