For the reaction sequence given below, the correct statement$(s)$ is$($are$)$
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$($In the options, $X$ is any atom other than carbon and hydrogen, and it is different in $P , Q$ and $R )$

• A
  $C-X$ bond enthalpy in $P , Q$ and $R$ follows the order $R > P > Q$.
• B
  $C-X$ bond length in $P , Q$ and $R$ follows the order $Q > R > P$.
• C
  Relative reactivity toward $S _{ N } 2$ reaction in $P , Q$ and $R$ follows the order $P > R > Q$.
• D
  $p K_{ a }$ value of the conjugate acids of the leaving groups in $P , Q$ and $R$ follows the order $R > Q > P$.