单选题评估以下语句： ALTER TABLE employees ADD CONSTRAINT employee_id PRIMARY KEY; 该语句将返回以下哪种结果（）A将返回语法错误B将向EMPLOYEES表添加约束条件C将改写EMPLOYEES表的一个现有约束条件D将启用EMPLOYEES表的一个现有约束条件
单选题
评估以下语句： ALTER TABLE employees ADD CONSTRAINT employee_id PRIMARY KEY; 该语句将返回以下哪种结果（）
A
将返回语法错误
B
将向EMPLOYEES表添加约束条件
C
将改写EMPLOYEES表的一个现有约束条件
D
将启用EMPLOYEES表的一个现有约束条件
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