If the coefficient of $x^r$ in the expansion of $(1+x+x^2+x^3)^{100}$ is $a_r$,and $S = \sum_{r=0}^{300} a_r$,then $\sum_{r=0}^{300} r \cdot a_r =$

  • A
    $(50) S$
  • B
    $(25) S$
  • C
    $(150) S$
  • D
    $(100) S$

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