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.