The third choice is appropriate.
an = 5 - 3(n - 1); all integers n ≥ 1
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This equation follows the form for the general term of an arithmetic sequence.
an = a1 + d(n - 1)
where a1 is the first term (corresponding to n=1), and d is the common difference. From the problem statement, a1 = 5 and d = 2 - 5 = -3.