JEE Math Practice Question

If $$f:R \to R$$ satisfies $$f$$(x + y) = $$f$$(x) + $$f$$(y), for all x, y $$ \in $$ R and $$f$$(1) = 7, then $$\sum\limits_{r = 1}^n {f\left( r \right)} $$ is

1. A.$${{7n\left( {n + 1} \right)} \over 2}$$
2. B.$${{7n} \over 2}$$
3. C.$${{7\left( {n + 1} \right)} \over 2}$$
4. D.$$7n + \left( {n + 1} \right)$$