Which two statements about subqueries are true? （）

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.