Which three statements about subqueries are true? （）

A main query can have more than one subquery.
B. A subquery can have more than one main query.
C. The subquery and main query must retrieve data from the same table.
D. The subquery and main query can retrieve data from different tables.
E. Only one column or expression can be compared between the subquery and main query.
F. Multiple columns or expression can be compared between the subquery and main query.