【单选题】What is the difference between a unique constraint and a primary key constraint?
A.
A unique key constraint requires a unique index to enforce the constraint,whereas a primary key constraint can enforce uniqueness using a unique key of or nonunique index
B.
A primary key column can be NULL,but a unique key column cannot be NULL
C.
A primary key constraint can use an existing index ,but a unique constraint always creates an index
D.
A unique constraint column can be NULL,but primary key columns can not be NULL