Given here is a pie chart representation of the  extent of global diversity of invertebrates. What groups the four portions (A-D)represent respectively?

Arthropoda is the largest phylum. Class Insecta (A) is the largest of Arthropoda. Mollusca (B) is the second largest among animals. The third one is occupied by crustaceans (C). All other animals together indicate the (D) portion of the pie chart.