Which of the following functions have a domain of all real numbers? f(x) = -8x^3 - 7
f(x) = |x| + 15
f(x) = -4
f(x) = 1/x
f(x) = 3x - 9
f(x) = cube root of x
f(x) = sqrt(x) + 3
f(x) = -2x^2 - 3x - 9