Choose the correct option for vapor phase refining.
$A :$ The metal should form a volatile compound with an appropriate reagent.
$R :$ The volatile compound obtained should be easily decomposable.

• A
  $A$ and $R$ are both correct and $R$ is the correct explanation of $A$.
• B
  $A$ and $R$ are both correct and $R$ is not the correct explanation of $A$.
• C
  $A$ is correct and $R$ is incorrect.
• D
  $A$ is incorrect and $R$ is correct.