In a face-centred cubic arrangement of $A$ and $B$ atoms,$A$ atoms are at the corners of the unit cell and $B$ atoms are at the face centres. If one of the $A$ atoms is missing from one corner of the unit cell,what is the simplest formula of the compound?

• A
  $A_{7} B_{24}$
• B
  $A_{7} B_{8}$
• C
  $A B_{3}$
• D
  $A_{7} B_{12}$