Which of the following is a geometric series?

A geometric series has a constant ratio between consecutive terms. In D, each term is half the term before it: 20 ÷ 40 = 1/2, 10 ÷ 20 = 1/2, and 5 ÷ 10 = 1/2. So 40 + 20 + 10 + 5 is geometric; the other choices do not have a constant ratio.

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