In the extraction of aluminium:
Process $X$: applied for red bauxite to remove iron oxide (chief impurity).
Process $Y$: (Serpeck's process) applied for white bauxite to remove $Z$ (chief impurity).
Then,process $X$ and impurity $Z$ are:

• A
  $X =$ Hall and Heroult's process and $Z = SiO_2$
• B
  $X =$ Baeyer's process and $Z = SiO_2$
• C
  $X =$ Serpeck's process and $Z =$ iron oxide
• D
  $X =$ Baeyer's process and $Z =$ iron oxide