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【单选题】
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 MODIFY CONSTRAINT stud _ id _pk PRIMARY KEY (student _ id);
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A.
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B.
按下［ Ctrl ］键的同时，敲击字母键
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D.
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