Conceptual: Which of the following is NOT a geometric sequence?
2,4,8,16,...2, 4, 8, 16, ...2,4,8,16,...
5,10,20,40,...5, 10, 20, 40, ...5,10,20,40,...
1,3,9,27,...1, 3, 9, 27, ...1,3,9,27,...
2,4,6,8,...2, 4, 6, 8, ...2,4,6,8,...