If $f: R \rightarrow R$ is defined as $f(x+y)=f(x)+f(y), \forall x, y \in R$ and $f(1)=10$, then, $\sum_{r=1}^n(f(r))^2=$

  • A
    $\frac{7}{2} n(n+1)$
  • B
    $5 n(n+1)$
  • C
    $\frac{50}{3} n(n+1)(2 n+1)$
  • D
    $\frac{100}{4} n^2(n+1)^2$

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