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单项选择题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   ADD CONSTRAINT stud_id_pk PRIMARY KEY (student_id);

E、ALTER TABLE students   MODIFY CONSTRAINT stud_id_pk PRIMARY KEY (student_id);

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