In the initial creation of a table, if a UNIQUE constraint is included for a composite column that requires the combination of entries in the specified columns to be unique, which of the following statements is correct?
a. The constraint can be created only with the ALTER TABLE command.
b. The constraint can be created only with the table-level approach.
c. The constraint can be created only with the column-level approach.
d. The constraint can be created only with the ALTER TABLE…MODIFY command.