Mathematics for IGCSE & O level - Sequences (Section 3 - No. 24)

Which of these sequence is *not* a geometric sequence?

1, 2, 4, 8,...

2, 6, 18, 54, ...

1, 3, 5, 7, ...

3, 9, 27, 81,...

Geometric sequences have a common ratio. A linear sequence does not have a common ratio.