The functions f(x) and g(x) are defined as: f(x) = f(x – 1) + x and g(x)
= g(x – 1) + f(x), where x is an integer greater than or equal to 1. If
f(0) = 0, then the value of k, for which g(k) – g(4) = g(10) – g(8), is

1) 6
2) 7
3) 8
4) 9
5) 10