Answer:
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The null set is the set that contains no elements. The only subset of the null set is the null set itself. The cardinality of the null set is 0.
Every nonempty set has at least two subsets, 0 and itself. The empty set has only one, itself. The empty set is a subset of any other set, but not necessarily an element of it.
The set A is a subset of the set B if and only if every element of A is also an element of B. If A is the empty set then A has no elements and so all of its elements (there are none) belong to B no matter what set B we are dealing with. That is, the empty set is a subset of every set.
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~~@Hershey
