Which two statements about subqueries are true? （）
- A A subquery should retrieve only one row.
- B A subquery can retrieve zero or more rows.
- C A subquery can be used only in SQL query statements.
- D Subqueries CANNOT be nested by more than two levels.
- E A subquery CANNOT be used in an SQL query statement that uses group functions.
- F When a subquery is used with an inequality comparison operator in the outer SQL statement, the column list in the SELECT clause of the subquery should contain only one column.