Let $f: R \rightarrow R$ be a function satisfying the condition $f\left(\frac{x+y}{k}\right)=\frac{f(x)+f(y)}{k}$, where $k \neq 0,2$. The function $f(x)$ is differentiable on $R$ and $f^{\prime}(0)=m$.
$f^{\prime}(x)$ is equal to
(A) $m$
(B) $2 m$
(C) $m+1$
(D) 0