请从下列对标识符(Key)的描述中选择不正确的选项
(A)标识符也成为键,根据其结构可以划分为单一键、组合键等。
(B)根据标识符的用途不同可以划分为主标识符、备用标识符、候选标识符等。
(C)一个实体中只允许定义一个标识符。
(D)实体的标识符允许包含其他实体中继承来的标识符。 答案解析: 一个实体中可以由多种标识符(键)
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