If the coefficients of the $r$-th,$(r+1)$-th,and $(r+2)$-th terms in the expansion of $(1+x)^n$ are respectively in the ratio $2:4:5$,then $(r, n) =$

  • A
    $(2, 7)$
  • B
    $(3, 8)$
  • C
    $(3, 9)$
  • D
    $(4, 9)$

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