Find a Formula for the Nth Term of the Sequence $9 , - 9,9 , - 9,9$

Multiple Choice

Find a formula for the nth term of the sequence.
- $9 , - 9,9 , - 9,9$ (9's with alternating signs)

A) $a _ { n } = 9 ( - 1 ) ^ { 2 n + 1 }$
B) $a _ { n } = 9 ( - 1 ) ^ { n + 1 }$
C) $a _ { n } = 9 ( - 1 ) ^ { 2 n - 1 }$
D) $a _ { n } = 9 ( - 1 ) ^ { n }$