What is the mechanism of marrow cavity enlargement in a long bone?

A) There is no mechanism; once formed, the marrow cavity does not change in size.
B) As bone matrix is added to the external surface, osteoclasts remove matrix in the center of the bone to enlarge the cavity.
C) Blood vessels entering the marrow cavity enlarge it by eroding away the inner surface of the bone.
D) The bone marrow cavity only increases in length, as the epiphyseal plates move apart.
E) None of the answers are correct.