JEE Math Practice Question

Let f : R $$ \to $$ R be a function which satisfies f(x + y) = f(x) + f(y) $$\forall $$ x, y $$ \in $$ R. If f(1) = 2 and g(n) = $$\sum\limits_{k = 1}^{\left( {n - 1} \right)} {f\left( k \right)} $$, n $$ \in $$ N then the value of n, for which g(n) = 20, is :

1. A.20
2. B.9
3. C.5
4. D.4