A subquery is called a single-row subquery when

A subquery is called a single-row subquery when

A.
the inner query returns a single value to the main query

B.
the inner query uses an aggregate function and returns one or more values

C.
there is only one inner query in the main query and the inner query returns one or
more values

D.
the inner query returns one or more values and the main query returns a single value
as output