Which of the following is an odd function?
f(x) = 3x2 + x
f(x) = 4x3 + 7
f(x) = 5x2 + 9
f(x) = 6x3 + 2x
f(x) = 3x2 + x
f(x) = 4x3 + 7
f(x) = 5x2 + 9
f(x) = 6x3 + 2x