Mark all statements that are correct. group of answer choices a union with the empty set is always empty. for any universal set, the complement of the empty set is nonempty. the empty set has no subsets. the empty set is a subset of every set. the empty set is a proper subset of every set. an intersection with the empty set is always empty. the cartesian product a × b is empty if a is empty or b is empty. the empty set has no proper subsets. the empty set, and only the empty set, has cardinality zero.