【单项选择题】
View the Exhibit and examine the structure of the EMP table.<br /> <img src="https://img.ppkao.com/2018-08/gelingxia/2018080114332223676.jpg" /><br /> You executed the following command to add a primary key to the EMP table：  <br /> ALTER TABLE emp  <br /> ADD CONSTRAINT emp_id_pk <br /> PRIMARY KEY （emp_id）  <br /> USING INDEX emp_id_idx；  <br /> Which statement is true regarding the effect of the command？（）<p> View the Exhibit and examine the structure of the EMP table.<br /> <img src="https://img.ppkao.com/2018-08/gelingxia/2018080114332223676.jpg" /><br /> You executed the following command to add a primary key to the EMP table：
A、A. The PRIMARY KEY is created along with a new index.
B、B. The PRIMARY KEY is created and it would use an existing unique index.
C、C. The PRIMARY KEY would be created in a disabled state because it is using an existing index.
D、D. The statement produces an error because the USING clause is permitted only in the CREATE TABLE command.
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