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

Question

( A ) $\frac{7n}{2}$
( B ) $7n(n+1)$
( C ) $\frac{7(n+1)}{2}$
( D ) $\frac{7n(n+1)}{2}$