In the Hoope's process for refining of aluminium,the fused materials form three different layers and they remain separated during electrolysis. This is because:

• A
  The upper layer is kept attracted by the cathode and the lower layer is kept attracted by the anode
• B
  There is a special arrangement in the cell to keep the layers separate
• C
  The $3$ layers have different densities
• D
  The $3$ layers are maintained at different temperatures