What was an early selective advantage of a coelom in

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