Write an Expression for the Apparent Nth Term of the Sequence

Multiple Choice

Write an expression for the apparent nth term of the sequence. (Assume that n begins with 1.) 9, 12, 15, 18, 21

A) $a _ { n } = 6 n + 3$
B) $a _ { n } = ( - 1 ) ^ { n } ( 3 n + 6 )$
C) $a _ { n } = 3 n$
D) $a _ { n } = 3 ^ { n } + 6$
E) $a _ { n } = 3 n + 6$