Given below is the representation of the extent of global diversity of invertebrates. What groups do the four portions $(a, b, c, d)$ represent respectively?

• A
  $a$-Insects,$b$-Crustaceans,$c$-Other animal groups,$d$-Molluscs
• B
  $a$-Crustaceans,$b$-Insects,$c$-Molluscs,$d$-Other animal groups
• C
  $a$-Molluscs,$b$-Other animal groups,$c$-Crustaceans,$d$-Insects
• D
  $a$-Insects,$b$-Molluscs,$c$-Crustaceans,$d$-Other animal groups