Determine whether each sequence is geometric or not. If it is geometric, enter the common ratio in the blank provided. If it is not geometric, enter None in the answer blank.

(a) 8, -4, 2, -1, . . .

Common ratio (or None) =

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(b) $\frac{1}{2}$, $\frac{1}{3}$, $\frac{1}{4}$, $\frac{1}{5}$, . . .

Common ratio (or None) =

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(c) 5, $\frac{5}{2}$, $\frac{5}{4}$, $\frac{5}{8}$, . . .

Common ratio (or None) =