Find a (non-standard) ordering of the set of natural numbers ℕ such that there exists a non-empty proper subset A ⊂ ℕ such that sup(A) exists in ℕ, but sup(A) is not in A.
a) 2, 1, 3, 4, 5, 6, ...
b) 1, 2, 3, 4, 5, 6, ...
c) 1, 3, 5, 7, 9, 11, ...
d) 2, 4, 6, 8, 10, 12, ...
