Which two statements about subqueries are true? （）
选项
A subquery should retrieve only one row.
A subquery can retrieve zero or more rows.√
A subquery can be used only in SQL query statements.
Subqueries CANNOT be nested by more than two levels.
A subquery CANNOT be used in an SQL query statement that uses group functions.
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.√