What was an early selective advantage of a coelom in animals?

What was an early selective advantage of a coelom in animals? A coelom _

A. Allowed asexual and sexual reproduction

B. Allowed cephalization and the formation of a cerebral ganglion

C. Contributed to a hydrostatic skeleton, allowing greater range of motion

D. Was a more efficient digestive system

Please explain why the correct answer is C, and not B