OCP的一个题目！不知道什么意思？

[saycool]

[Q][EXAM-001]"A non-pair wise comparison produces a cross product.“什么意思？

Which statement about multiple sub-queries is True?
A. A pair wise comparison produces a cross product.
B. A non-pair wise comparison produces a cross product.
C. In a pair wise subquery, the values returned from the subquery are compared
individually to the values in the outer query.
D. In a non-pair wise subquery, the values returned from the subquery are
compared as a group to the values in the outer query.

Answer: B
Explanation:
Answer B is correct because a non-pair wise comparison really produces a cross product.

这几个选项到底是什么意思啊？