In the following pie chart of global invertebrate diversity,what do $A$ and $B$ represent?

• A
  $A$ - Molluscs,$B$ - Crustaceans
• B
  $A$ - Insects,$B$ - Molluscs
• C
  $A$ - Crustaceans,$B$ - Molluscs
• D
  $A$ - Molluscs,$B$ - Insects