What is the explicit equation for the following geometric sequence? 18, 9, 4.5, 2.25,...

a
\large a_n=\left(18\right)2^{n-1}

b
\large a_n=\left(18\right)-2^{n-1}

c
\large a_n=\left(18\right)-\frac{1}{2}^{n- 1}

d
\large a_n=\left(18\right)\frac{1}{2}^{n-1 }

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