Which of the following sequences are geometric?
2,4,8,16,...2, 4, 8, 16, ...2,4,8,16,...
3,9,27,81,...3, 9, 27, 81, ...3,9,27,81,...
1,−1,1,−1,...1, -1, 1, -1, ...1,−1,1,−1,...
1,2,4,7,...1, 2, 4, 7, ...1,2,4,7,...