Let $N$ be the set of positive integers. For all $n \in N$, let $f_n=(n+1)^{1 / 3}-n^{1 / 3} \text { and }$ $A=\left\{n \in N: f_{n+1}<\frac{1}{3(n+1)^{2 / 3}} < f_n\right\}$ Then,

  • [KVPY 2019]
  • A

    $A=N$

  • B

    $A$ is a finite set

  • C

    the complement of $A$ in $N$ is nonempty, but finite

  • D

    $A$ and its complement in $N$ are both infinite

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