Which of the following is a geometric sequence?

A geometric sequence has the same ratio between each pair of consecutive terms. In choice D, each term is multiplied by 3: 3 ÷ 1 = 3, 9 ÷ 3 = 3, 27 ÷ 9 = 3, and 81 ÷ 27 = 3. So 1, 3, 9, 27, 81, . . . is geometric.

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