Which two statements are true about the primary key constraint in a table？ （）
A. It is not possible to disable the primary key constraint.
B. It is possible to have more than one primary key constraint in a single table.
C. The primary key constraint can be referred by only one foreign key constraint.
D. The primary key constraint can be imposed by combining more than one column.
E. The non-deferrable primary key constraint creates an unique index on the primary key column if it is not already indexed.
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