Fact 1: Echinoderms have a unique water vascular system with tube feet. Fact 2: Another group of echinoderms are the echinoids. Given the two facts above, which has a unique water vascular system?
Possible answers: (a). roots; (b). trees; (c). Conifers; (d). Echinoids; (e). fur seals; (f). Most plants; (g). ferns; (h). bushes;
(d).