单选题：学生表中有"学号"、"姓名"和"年龄"三个字段，SQL语句"SELECT 学号 FROM 学生"完成的关系操作称为（　　

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题目内容：
学生表中有"学号"、"姓名"和"年龄"三个字段，SQL语句"SELECT 学号 FROM 学生"完成的关系操作称为（　　）。 A．选择
B．投影
C．连接
D．并

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