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The following sets are all empty sets, but they can be. . .
D should be chosen for this question, because the formula in D can calculate x=-2 or 1/2, which are not natural numbers N, so D is an empty set. A, B and C are sets with O, empty set and (0,0) as elements respectively. They are not empty sets, so the answer should be D.

I hope what I said can help you, hehe, good luck!