Which of the following statements accurately explains the requirement for the "each GROUP BY expression must contain at least one column that is not an outer reference" error in SQL?
A. It indicates that every column in the GROUP BY clause must refer to a column in the outer query.
B. It specifies that each GROUP BY clause must include at least one column from the SELECT statement.
C. It means that every expression in the GROUP BY clause must include at least one column from the tables being directly queried, not just from a correlated subquery.
D. It mandates that each GROUP BY clause must contain only columns that are part of the table's primary key.