Which of the following sequences can be described as an arithmetic sequence? (Select all that apply.)
a) { 9, 10, 12, 15, 19, ...}
b) { -9, -3, 3, 9, 16, ...}
c) { 7, 1, -5, -11, -17, ...}
d) { 7, 15, 23, 31, 38, ...}
e) { 6, 4, 2, 0, -2, ...}
f) { -10, -30, -90, -270, -810, ...}