Question: Which of the following statements about SQL constraints and data integrity is correct?

A) A UNIQUE constraint allows duplicate values in a column, provided that the duplicates do not appear in the same row.
B) A FOREIGN KEY constraint ensures that a column or a group of columns in one table matches values in another table, maintaining referential integrity.
C) A CHECK constraint can only be applied to numeric columns to enforce specific conditions on the data being entered.
D) A PRIMARY KEY constraint can be defined on multiple columns, but it does not allow any NULL values in any of those columns.