Many Sponges, Cnidarians, and Echinoderms Have Few If Any External

Many sponges, cnidarians, and echinoderms have few if any external characteristics distinguishing males from females. This seemingly odd structural characteristic is most likely associated with which of these functions?

A) substrate feeding
B) reproducing asexually only
C) reproducing by spawning (simultaneous release of gametes into the water for fertilization)
D) having a gastrovascular cavity that takes up most of the internal space.