The Following Questions Refer to the Paragraph Below

The following questions refer to the paragraph below.
A farm pond, usually dry during winter, has plenty of water and aquatic pond life during the summer. One summer, Sarah returns to the family farm from college. Observing the pond, she is fascinated by some six-legged organisms that can crawl about on submerged surfaces or, when disturbed, seemingly "jet" through the water. Watching further, she is able to conclude that the "mystery organisms" are ambush predators, and their prey includes everything from insects to small fish and tadpoles.
-Protostomes that have an open circulatory system and an exoskeleton of chitin are part of which phylum?

A) Cnidaria
B) Annelida
C) Mollusca
D) Arthropoda
E) Echinodermata