$A$ coin is biased so that the head is $3$ times as likely to occur as tail. This coin is tossed until a head or three tails occur. If $X$ denotes the number of tosses of the coin,then the mean of $X$ is

  • A
    $\frac{21}{16}$
  • B
    $\frac{81}{64}$
  • C
    $\frac{15}{16}$
  • D
    $\frac{37}{16}$

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