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The formula for any geometric sequence is an = a1 · rn - 1 , where an represents the value of the n th term, a1 represents the value of the first term, r represents the common ratio, and n represents the term number. What is the formula for the geometric sequence 4, 8, 16, 32, ...?

A. an = 4 · (1/2) n - 1
B. an = 2 · 4 n - 1
C. an = 32 · 2 n - 1
D. an = 4 · 2 n - 1