Find a20, the 20th term, of the arithmetic sequence 1, 3, 5, 7, 9, …

The sequence starts at 1 and increases by 2 each time, so its nth term is aₙ = 1 + (n − 1) × 2. For n = 20, a₂₀ = 1 + 19 × 2 = 39. The 20th term is 39, choice A.

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