Why Do Adult Urochordates Tunicates) Lack Notochords, Even Though Larval

Why do adult urochordates tunicates) lack notochords, even though larval urochordates have them?

A) Larvae use notochords to induce tissue differentiation; in adults, tissue is already differentiated.
B) Larvae use notochords to organize their nervous systems; adults' nervous systems are fully developed and do not change.
C) Larvae use notochords to stiffen their bodies; in adults, the notochord is replaced by a column of bone.
D) Larvae use notochords to aid in swimming; adults are sessile and, thus, no longer need notochords.