Which of the following statements is correct?
a. If the subquery returns only a NULL value, the only records returned by an outer query are those containing an equivalent NULL value.
b. A multiple-column subquery can be used only in the outer query’s FROM clause.
c. A subquery can contain only one condition in its WHERE clause.
d. The order of columns listed in the SELECT clause of a multiple-column subquery must be in the same order as the corresponding columns listed in the outer query’s WHERE clause.