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You need to modify the STUDENTS table to add a primary key on the STUDENT_ID column. The table is currently empty.Which statement accomplishes this task?（）
A:ALTER TABLE students ADD PRIMARY KEY student_id;
B:ALTER TABLE students ADD CONSTRAINT PRIMARY KEY (student _ id);
C:ALTER TABLE students ADD CONSTRAINT stud _ id _pk PRIMARY KEY (student _ id);
D:ALTER TABLE students MODIFY CONSTRAINT stud _ id _pk PRIMARY KEY (student _ id);

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出自：Oracle认证

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