Answer :
Its means that the addition of natural numbers obeys the closure property.
In mathematics, the closure property is a property that states that if a set is combined with all its limit points, then the combined set is also a limit point. The closure property applies to sets of real numbers, vectors, and matrices. It states that the closure of a set is itself, i.e., the set is closed under the given operation. The phrase "closed under addition" means that when two natural numbers are added together, the result is also a natural number. In other words, natural numbers are part of a closed system concerning addition; the addition operation always produces another natural number.
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