If a constraint-based set is defined as 'the set of all x such that x is a natural number and x < 10', which of the following is a valid representation of this set?
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{1, 2, 3, 4, 5, 6, 7, 8, 9}
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{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
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{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
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{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}