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View the Exhibit and examine the structure of the EMP table. You executed the following command to add a primary key to the EMP table：   ALTER TABLE emp   ADD CONSTRAINT emp_id_pk  PRIMARY KEY （emp_id）   USING INDEX emp_id_idx；   Which statement is true regarding the effect of the command？（）
A: The PRIMARY KEY is created along with a new index.
B: The PRIMARY KEY is created and it would use an existing unique index.
C: The PRIMARY KEY would be created in a disabled state because it is using an existing index.
D: The statement produces an error because the USING clause is permitted only in the CREATE TABLE command.

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出自：Oracle认证

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