Find an Explicit Rule for the Nth Term of the Sequence

Multiple Choice

Find an explicit rule for the nth term of the sequence.
-The second and fifth terms of a geometric sequence are $- 16$ and 128 , respectively.

A) $a _ { n } = 8 \cdot ( - 2 ) ^ { n + 1 }$
B) $a _ { n } = 8 \cdot 2 ^ { n }$
C) $a _ { n } = 8 \cdot ( - 2 ) ^ { n - 1 }$
D) $a _ { n } = 8 \cdot 2 ^ { n - 1 }$