Which two statements are true regarding subqueries?

Which two statements are true regarding subqueries? (Choose two.)

A.
Only two subqueries can be placed at one level.

B.
A subquery can be used to access data from one or more tables or views.

C.
If the subquery returns 0 rows, then the value returned by the subquery expression is NULL.

D.
The columns in a subquery must always be qualified with the name or alias of the table used.

E.
A subquery in the WHERE clause of a SELECT statement can be nested up to three levels only.