Identify each sequence below as geometric, arithmetic, or neither.
(a) 1, 3, 5, 7, 9, 11, 13, 15
(b) 21, 16, 12, 9, 7, 6
(c) .3, .03. .003, .0003, .00003
(d) -4, -12, -36, -108, -324