Consider the integers with the normal operation of addition.

On which of these subsets of the integers is addition "closed" ? (select all that are correct)

a)
The natural numbers: {1, 2, 3, ... }

b)
The multiples of 3: {..., -6, -3, 0, 3, 6, ...}

c)
The odd integers: {..., -3, -1, 1, 3, 5, ...}

d)
The nonnegative integers less than 5: {0, 1, 2, 3, 4}

Question

Consider the integers with the normal operation of addition.

On which of these subsets of the integers is addition "closed" ? (select all that are correct)

a)	The natural numbers: {1, 2, 3, ... }

b)	The multiples of 3: {..., -6, -3, 0, 3, 6, ...}

c)	The odd integers: {..., -3, -1, 1, 3, 5, ...}

d)	The nonnegative integers less than 5: {0, 1, 2, 3, 4}

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