Four of the Following are Echinoderms

Question

Four of the following are echinoderms.Which one is the exception?  
A) Sea cucumber
B) Sea star
C) Urchin
D) Brittle star
E) Krill

Accepted Answer

Krill is not an echinoderm, it is a crustacean. The other options (sea cucumber, sea star, urchin, and brittle star) are all echinoderms.